User blog comment:Dtuck010/How Old Is Bart Allen?/@comment-63.135.255.27-20120824132110

In the original comics, Bart was technically two years old. His heightened metabolism somehow made him age unnaturally fast and by then he already looked like he was twelve. He had been placed in a virtual reality to allow him to grow at that rate without going insane. Sometime later, he entered a speed burst that forced his metabolism into normality.

In the series, he seems about fourteen or fifteen, but his past (technically the future) hasn't been explained yet, though he clearly has many secrets and acts in a way more similar to his adult self in the comics than his teenaged self, who treated life 'like a video game'.