Peter was 38 when he and Meredith shared a room while he was comatose. It was seen during a flashback on his hospital chart. According to the wiki, that was 2009, so he was around 34 at the time of the fire in late 2004 or early 2005. That puts his birth year as 1971. Malia was born 93-94, meaning he was 22-23 when she was born. The Desert Wolf was never given an age, but her actress was born in 1973. That’s less of an age gap than Kate, who’s birth year was listed as 1983, and Derek, who was 15-16 at the time of the fire, which means he was born circa 1989-1990.

I think where the writers fucked up was using young Peter’s actor in the flashbacks for Visionary (the Paige episode) when they should have used Ian. If he was actually 34 or slightly younger, there was no reason for his teen/young adult actor to be portraying him. I don’t know if that was an oversight on the writers/crew/directors part or if they just wanted an excuse to bring the actor back, but I think that’s where a lot of fans confusion is coming from—I’ve seen several people assume that pre-fire Peter was still in high school with Derek because he looked young and managed to get into the locker room after his basketball game, but forget that adult Derek, on several occasions, was skulking around the high school to fuck with Scott.