I was watching the season 1 episode "Boy Meets Girl" and when Shawn is telling Cory and Eric about how his second date with Hilary didn't go well, he refers to himself as a "thirteen year old burnout." Now, in season one, Cory was 11 for the majority of the season but he turned 12 in the episode "Kid Gloves" which is two episodes before the episode I mentioned before. So Cory was 12 at the time but Shawn referred to himself as being 13. Not only that, in the season 3 episode "He Said, She Said" Mr. Turner says that Shawn is 15 and in the episode before that "What I Meant To Say" Topanga says that she and Cory are 14 years old. So is Shawn maybe about a year older than Topanga? It might be possible he got held back a year.