This is by far the youngest Opening Day roster coach Bill Belichick has assembled with the Patriots. The average age of New England’s roster right now (the team could have added four more tight ends late Sunday night) is 25 years, 274 days, according to STATS, LLC. That’s more than a year younger than the 2010 squad (26-280). The third-youngest team was Belichick’s first, the 5-11 juggernauts of 2000 (26-329).

Where the youth really shows up is on defense. The average age of the starters is 25. The only other Belichick team to be under 27 was the ’10 version at 25.1, according to numbers from Pro-Football-Reference.com.

Average Opening Day ages for the championship teams: 27-177 (’01), 28-120 (’03), and 27-194 (’04). Defenses: 28.7, 30, 28.7.

That means, basically, the rebuilding effort started in 2007 on defense with the drafting of Brandon Meriweather (now on his third team with the Redskins) was not overly successful, and they are in another phase of it this season. At least the Patriots have some, in theory, stud pieces to do it with two first-round picks (end Chandler Jones and linebacker Dont’a Hightower) and second-round pick Tavon Wilson. They are also getting Ras-I Dowling back at nickel cornerback as basically another high pick after his rookie season was washed out. It looks like this phase should work on paper, but we’ll have to see it on the field.

Why did the Patriots, in what used to feel like was an “all-in, go-for-it, cover-every-base bid to get another Super Bowl title” season, decide to part with experience now? Part of it was the key older players they cut (Dan Koppen, Deion Branch, James Ihedigbo, Bobby Carpenter, and Brian Hoyer) either weren’t better than the other players, or the difference wasn’t that great, so why spend more money?

I also think part of it was the Patriots know — more than most teams — that there is going to be a salary-cap squeeze in ’13 and ’14 before the cap starts to get larger in ’15. The average age of the cut veterans was 29.6. In base salary, they were due a combined $5.1 million this season.

The replacements (Nick McDonald, Greg Salas, Nate Ebner, Mike Rivera, and Ryan Mallett) average 24.2 years, and are due to make a combined $2.23 million this season. Under the new CBA, extra cap money can be rolled over every season.

Of course, the Patriots have only themselves to blame for having to watch their spending a little because of their own drafting (2006-09) and consequent free agent whiffs.

Between Chad Ochocinco and Shaun Ellis last year, and Jonathan Fanene, Robert Gallery, Koppen, Branch, Jabar Gaffney, Joseph Addai, Tony Fiammetta, Carpenter, and Ihedigbo, the Patriots spent nearly $11 million in signing bonuses and got absolutely nothing in return.

The Patriots hit on some that were very cost effective, like Andre Carter and Mark Anderson, but $11 million is a big bagel to swallow. That’s money wasted when if they were just a tad better at drafting and didn’t have to rely on free agent retreads, they could have saved a ton of cash and reinvested that in the roster (Wes Welker anyone?).

Free agency is fool’s gold. You’re trolling in other people’s trash, and you can make a lot of mistakes, especially on medical evaluations. Have you noticed how many ex-Patriots have flamed out in other places? Teams don’t let good players go as much anymore, since nearly all the teams are good at managing the cap now.

You know how many “big” free agents are going to start on the Patriots this season? Two: Brandon Lloyd and safety Steve Gregory. Every other starter is homegrown. Draft, develop, and reward. The Patriots finally have their system where they want it after the last three drafts, which look like they’re going to be bountiful. It really sets up the team down the road.

When the media brings up how poorly the Patriots drafted for a stretch, defenders always counter it by saying that it hasn’t hurt the Patriots. In regular-season wins and losses, no it hasn’t. Thank goodness for Belichick, Tom Brady, and AFC East because the combination of the drafting and free agency blunders would absolutely cripple most teams.

The thing is, it was even puzzling the Patriots brought back some of those guys (specifically Koppen, Branch, and Ihedigbo) when it wouldn’t hurt them to go younger. On top of that, the Patriots gave them signing bonuses. Puzzling.