Why are these people smiling?

Life begins at 40. Don't trust anyone over 30. Fifty is the new 30. Sixty is the new 40.

Hey, guys, what about 49? Gals, what about 45?

These are the turning-point ages, according to a new study published by the Cambridge-based National Bureau of Economic Research, when American men and women, respectively, are least happy. Not necessarily sad, but least happy. After that come some 15 years of increasing happiness. Authors David Blanchflower and Andrew Oswald call it the U-shaped curve of well-being. Discovering the whys behind the pattern, they write, "seems an important task for future research."

Herein is our modest contribution. We asked three 49-year-old men and three 45-year-old women -- people in the statistical trough of that U -- for their reflections.

IRENE SEGE

Jenny Rudolph

"I'm sort of at a crux moment right now, trying to figure out what my direction will be for the next 15 years. . . . I feel like identity-wise I'm comfortable with who I am. Financially things are running smoothly. There's a very solid platform to build on."

Prominently displayed on a bookshelf in Jenny Rudolph's office at Boston University's School of Public Health is the book "Change the World" by Robert E. Quinn. The subtitle is "How Ordinary People Can Accomplish Extraordinary Results." Her license plate reads "IGNITE," which sums up her mission, professionally, academically , and personally, of igniting people to "find their authentic voice and power."

Rudolph's a lesbian and mother who lives in Arlington and shares custody of her 10-year-old daughter with her ex and plans to adopt a child with her current partner. She recalls the "very interesting journey" of "being socially disapproved as a lesbian and socially approved as a mom." She's a Buddhist who tries to practice the mindfulness of her adopted faith, an academic who tries to apply what she reads to her own life.

"One of the really profound things for me was can you tolerate being your own person which is different from what your parents want, what your partner wants. Holding onto your own true identity even if it's not what someone else wants," she says.

"What's important to me happiness-wise, " she adds, "is tolerating the consequences of your actions, rather than trying to deny or sugarcoat. I do think age has something to do with it. It gives me fewer and fewer excuses for not living the life that's most meaningful for me."

Steven Thomas

"People have called me a glass-half-empty person. That's because I'm a problem solver. You can't look at a half-empty glass without seeing it needs to be topped off. You tend not to be content most of the time because life isn't perfect."

On the phone in his State House office when a visitor arrives, Steven Thomas, the Senate's deputy counsel, is fielding a question about balancing federal prerogative with a new state law. Before he went to law school, Thomas handled different sorts of problems as a computer programmer. In the last few weeks -- "it came like a bolt," he says -- he's been thinking it's time to start seeing that glass as half full. It's what his wife, who's battled breast cancer, often reminds him to do.

Thomas and his wife have a 10-year-old daughter and live in Westwood. Every year, Thomas arranges pictures of them in a custom-made wall calendar. The latest, which shows his daughter blowing a bubblegum bubble on April's page, hangs on the wall of his office. Older ones lie in a pile on a shelf.

"We have so much. It's easy to walk around complaining about the minutiae," Thomas says. "It's like if you're driving and the seat in the way back is squeaking, you're not paying attention to the wonderful leather or the nice sound system. Sometimes you just have to turn up the radio and sing along and ignore the squeak."

More profiles, Page D6

Paul Cartwright

"You're getting close to your retirement and your kids are out of the house. You're getting back to your single life again. That's how I look at it."

Paul Cartwright pulls his postal truck up to a bank of mailboxes on a bucolic lane in Concord lined with multimillion-dollar manses set far back from the road. He's a criminal justice major and former corrections officer who opted for the reduced stress and better hours of being a letter carrier. For fun and extra money, he officiates at high school soccer and basketball games.

Soon his Acton home becomes an empty nest. He's just back from taking his daughter, a high school senior, to look at Saint Joseph's College in Maine, where she plans to study nursing. His son, 22, expects to graduate from the University of Massachusetts in Amherst in another semester.

"You get to enjoy seeing what they'll be doing with their lives. Excitement to see what turns out, you know what I mean?" Cartwright says. "Happiness has to do with stress. Worries with the kids. What they're going to do. How they're going to live on their own. The bills. Plus the emotional stress. Look at me. Criminal justice major and I'm delivering mail. Hopefully they can make it in life and be happy."

He worries -- but not as much as his wife, he says -- about having enough money for a comfortable retirement, to travel with friends, to handle healthcare costs. He points to the houses on his route. "The people living here," he says, "have the same worries I have, on a different scale."

What are the ingredients of happiness? "Family," he says. Next? "You hate to say money over friends." He pauses. "I would say friends. Then money. The quality of life. If I had all the money in the world and didn't have any friends, that wears pretty thin."

Lynda Kirby

"There's just one thing that can make me happier. I don't have my own home yet. That's going to happen this year. When that happens, I'm good."

Lynda Kirby, assistant to Brookline's assistant superintendent of schools, has been working in the school system from which she graduated for 20 years. She and her second husband (her first marriage ended in divorce) and their two children, ages 9 and 11, live in an apartment in her father's house in Dedham. Everyone, thank God, is healthy. But life's hectic and money tight.

"I have a lot of friends that I graduated with," Kirby says. "We're all in the rat race."

Blanchflower and Oswald, like other researchers, find that people with money are happier than people without, but they also find that getting older, after the mid-to-late 40s, has a statistically more positive effect on happiness than doubling income. Still, Kirby would be happier with the things, like a house or the chance to work only part time , that money can buy.

"There were days when these kids were babies, I can remember standing in the kitchen with my winter coat still on, my pocketbook still on and putting on the pot for spaghetti, with one at each leg and thinking, oh my God, I'm miserable," she says. Her daughter has a learning disability, which means long nights of helping her with homework. "The laundry's not getting done last night. And we got take-out," Kirby says. "That's what we do. We work for them. It's fine. I just need a few more hours added on to each day."

Back in her early 30s, Kirby longed for a family. "I didn't consider myself fulfilled and happy until I was married with children," she says. "Once my kids are in college I can socialize again," she adds. "Not that I want them to go, but it's all right. Time for me again. Working full time and raising kids is the hardest thing you can do."

Timothy Lamson

"When you have a great relationship with your wife and healthy kids and you look at everything that's going on in the world, I don't see how you can't be upbeat and happy."

The mementos in Timothy Lamson's office in Newton overlooking the Massachusetts Turnpike -- the photograph from last year's family ski trip in Utah, the photo from a family kayaking trip in Maine, the coffee mug imprinted with a family picture from a Cape Cod whale watch -- are souvenirs of a good life.

So when Lamson is told about the U-shaped graph of happiness, he says, "I couldn't disagree more." He's vice president of Unidine, a food service company, and lives in Amesbury with his wife and their two teenage sons, the elder of whom heads to college in September. He and some buddies shared a ski house in Tahoe after college; then he met his wife, and it's gotten better ever since. "It's work," he says. "It's a journey. It's not a gift with a big bow."

Lamson approaches his 50th birthday with caution.

"I'd rather be turning 25," he says. "You start feeling older when people around you get ill." In the past year, a few friends were diagnosed with terminal cancer and two business associates died of heart attacks. All were in their early 50s.

"I'm going to be there this year," he says. "It drags you to that gym every afternoon."

His gym bag is in his car. "I think it's a big part of being happy. Being fit," he says. "My father-in-law is in his 70s. He skis every day in the winter. Plays golf in the summer. He's my idol."

Digna Gerena

"In my 20s I was still discovering who I was. You were more concerned with what other people expected of you, not what you wanted to do. Not just your parents, but the guys that you liked. There are a lot of ways to get what you want out of your life without putting yourself in a box."

Digna Gerena calls herself "happily divorced." That's not what she was feeling back in her 20s, when she had two young children and her husband told her he wanted out of the marriage.

"At 45, I'm thinking of all the stuff I learned. You develop not just skills but survival skills. It's a second time around. You know you can get through things," she says. "I'm lucky that I'm healthy and haven't had any catastrophe financially."

Gerena, who lives in East Boston, is program manager in the training and workforce development office at Massachusetts General Hospital. She also produces an online show, "Boston Latino," for the Boston Neighborhood Network. Her children are 21 and 24, and in a few months she'll be a grandmother.

"I try not to define myself by what my kids and parents do. You inherit labels you didn't earn yourself," she says. "I'm not going to be a conventional grandmother," she adds, "but I did buy two rocking chairs for my deck."

Chances are she won't be available for much babysitting. Now that Gerena's children are grown, she's focusing more of her time on community activities -- her work, for instance, on the board of the Latino Professional Network.

"I'm happily divorced. I'm open to something, but it's not like in your 20s. My life is so complete right now it will only add to what I have. It will complement. It won't be essential," Gerena says. "Forty was somewhat liberating for me. I already had my kids. I already bought a home. I was at a good place in my career. I was knowing who I was. The next 15 years, if I continue to enjoy good health, I don't feel are going to be as demanding of me."