A fan on Twitter recently asked showrunner Marlene King to clear up this mystery. Sadly, she responded with a less-than-helpful answer: "Wren is old enough to know better and young enough not to care."
Well, that might explain how Wren behaves, but not his actual age. However, we know he's a doctor. Assuming that he didn't skip any years of school, that means he must have spent four years as an undergrad, and then another four in med school. If he's still doing his internship in Rosewood, that would make him eight to nine years out of high school, or about ten years older than the girls.
Or, as Marlene says, still young enough not to care!
Catch an all-newepisode of PLL on Tuesday, June 19 at 8 p.m. ET/PT onABC Family.