Anyone who feels guilty playing Pikmin has confidence issues. It's a GREAT game in which you should feel no shame. I'm 16 and I'm perfectly secure playing pokemon, pikmin, super princess peach, or any other game that seems "kiddie". As long as it's a good game I'll play it without shame.
Anyway, that being said, my guiltiest pleasure is probably Naruto. I read every chapter (manga), then I started reading other mangas that are infinitely better (you know, the ones with good plots instead of random and generic plots meant to facilitate violence), and now I really don't think I should still be reading naruto every friday, but I'm
1. Caught on the plot cliffhangers
2. Peer pressured into it because some of my friends still like it