Wait, how old is Barry supposed to be? Because the episode said the flashback was "eighteen years ago," so bb!Barry is nine? Ten?
Barry was born circa 1989 (he’s eleven-years-old as of March 18, 2000, the night Nora Allen was murdered). Oliver takes Stephen Amell’s birthday, so it’s reasonable to assume Barry takes Grant’s (January 14). Note: if Barry’s birthday falls after March 18, then he was born in 1988. Either way, he was born in either 1988 or ‘89.
If we go the Grant route (Barry’s b-day: Jan 14 1989), then Barry’s 28 as of March 21, 2017. “Eighteen years ago” is Barry as a ten-year-old.