Actor Mark Wahlberg knows a thing or two about bad-behaving teenagers. So when Dorchester’s most famous former juvenile delinquent speaks, Justin Bieber should probably listen. In London to promote his new movie, “2 Guns,” Wahlberg (inset top) was asked what counsel he would give to Bieber, who can’t seem to stay out of trouble. (In the most recent scrape, Bieber (inset bottom) and his entourage were involved in a weekend brawl in a Hamptons nightclub.) “Justin, are you listening? Don’t be so naughty, yeah?” Wahlberg said during a webchat. “Be a nice boy, pull your trousers up, make your mom proud and stop smoking weed, you little [expletive].” Wahlberg, who served time as a teen for assault, said media scrutiny is more intense now. “He’s a teenager living in a different day and age. . . . “He’s a nice enough kid and you’re going to be a teenager, but if you’re being a teen in the spotlight you’re going to be criticized for it. This career can be short-lived — you might as well be the best you can while you’re doing it.” Wahlberg also revealed he and Bieber are talking about doing a movie together. Or at least they were.