I'm twenty six, and didn't go to my first con until after I had finished college.
Your age isn't really a big deal - the average anime con crowd tends to be young, certainly, because most Americans weren't exposed to anime until the early 2000's. Conversely, go to a scifi convention, and the average attendee is closer to fifty, and often still dressed up.
I guess the only thing is to be vaguely conscious of your age; a fifty year old cosplaying as a fourteen year old *may* send out the idea of a peter pan complex, if not executed with sufficient grace. However, at 25, there's really no such issue to worry about.
At Akicon this past year I met a seventy year old women cosplaying as Grandma Sophie from Howl's Moving Castle; at Ohayocon, I met a forty year old mother of two dressed as Mom Lalonde from Homestuck, with her kids dressed as Rose and Dave. In both cases, it was amongst the most adorable things I've ever seen.