It’s hard to imagine Paul Westerberg, the guy who’s written so many stellar songs about adolescence, being a grown-up. But that’s what he is. The 56-year-old former Replacements frontman is the proud father of a college-bound freshman. Friday, Westerberg’s ex, author-musician Laurie Lindeen, posted a picture of the couple’s son, Johnny, with the caption: “Senior decision day!” In the photo, Johnny, who bears a striking resemblance to his rocker dad, is wearing a Colorado State University hoodie. Westerberg, who did a series of shows last year with the reunited Replacements, recently released a CD with Juliana Hatfield, calling themselves The I Don’t Cares.