Kin Gordon from Sonic Youth was in her mid to late twenties when she picked up the bass. She's done alright for herself.
Wow...they have some nasty guitar, not sure how she's looked upon as a bass player though?
Listening to some of the pop-rock bands, a lot of songs can be learned in 5 minutes and it doesn't take much skill to play them. Don't see why someone couldn't become a successful musician starting in their twenties. Looking at the people who are really considered great though, I'd bet most started young.