Something about the job that requires a swimsuit says “Not quite an adult yet.” Lifeguards in their thirties (“I’m 29.”) are screenwriter shorthand for “didn’t sell out” (Sam Elliott in “Lifeguard” in the ’70s) but also never grew up — stuck — immature, living for the moment too much. Kristen Bell takes on that sort of character in “The Lifeguard,” that rare movie that doesn’t require her to be a light comedienne. Amy Madigan as her mom? Great casting. Bell’s character gets to face the music, but justify a generation’s ennui about moving back home and not being able to suck it up and get on with it. A movie full of possibilities.