Tweens and teens didn’t light up the frame for “Green Lantern” this weekend. But a solid Monday perf totaling $5.1 million — almost double its nearest competitor — suggests the Warner Bros. comicbook pic is heating up with under-25 auds.“Green Lantern” opened to $53.2 million through Sunday, of which moviegoers over 25 contributed the biggest share at 63%; pic so far has tallied $58.3 million. Now, Monday’s perf indicates that the pic’s age gap appears to be shrinking. According to B.O. observers, that’s because auds under 25 (and particularly those under 18) are able to boost midweek sales, especially during the summer when school is out. “I guarantee we’re getting younger,” said Warners distribution exec Dan Fellman. “If you look at the comparisons of what the other films did, you’ll find that we held our own.” Fellman said biz should slow down Tuesday tonight, but pick up later in the week as younger auds turn out for daytime screenings, boosted by positive word of mouth. (“Green Lantern” scored a B+ CinemaScore rating among under 18 auds vs. its overall B.) Paramount’s “Super 8″ was the No. 2 pic on Monday, grossing $2.6 million, followed by 20th Century Fox’s “Mr. Popper’s Penguins,” which tallied $2.4 million in its first post-debut foray. So far, “Super 8″ has grossed $75.6 million; “Penguins,” $20.8 million.