Look at Upper Queen Anne, that is probably my favorite area of Seattle besides where I live.
Also, Fremont is full of people in their 20's. Mostly recent college grads, or some are still in College.
First Hill can get a little sketchy, but if you stick with anything under 10th Ave you should be fine.
Belltown has gone downhill in the past few years. Its not really full of hipsters, but mostly douchebags and old guys wearing Affliction and Robert Graham shirts trying to...