I'll be 25 when school starts. Temple definitely has an older vibe... but not old! Which is good!
Center City is expensive (its no NYC or DC though, thank god).. 1br and studios are expensive, however if you can get a roommate you can something alot nicer. Fortunately its me and my fiancee in our 1br so we don't mind center city prices.
In reality a place in old city or northern liberties isn't all that more inconvenient than center city. When you think about it, most likely you aren't going to walk to school, so whats an extra 3-5 minutes on the subway or the el.