I'll lead by saying that's not a fetish of mine, but I kind of get it, and in a way that has nothing to do with age.
If a person who is attracted to girls is surrounded by almost exclusively girls in school uniforms during several formative years of their life in which their sexual interests are initially developing, is it that strange they'd grow to find the uniform attractive?
Maybe the fact that I didn't go to a school with uniforms is part of the reason I don't share that particular taste.
Personally I always thought it was just because it looked good, like it's stylish and esthetically pleasing. But now as an adult I realize that for a lot of people it is about that connection to younger girls. Gross.