Possibly Pokemon... I'm 26 and still watch it. But then again, I'm also involved with this: http://www.amitysquare.co.uk so it kind of explains it a little, right?
The Big Bang Theory is a recent guilty pleasure. It's far too close to my real experiences with friends at Uni and so forth, and some of the time I feel slightly shameful to get the geek references over the pop culture ones, but it's good fun.
Oh, and there's Densha Otoko...
Geeky guy saves stunning female from groper on train, and ends up trying to get together with her via the help of 2chan. It's charming and supposedly true story.