For me, age does not matter. I've played with people up to the age of 90. I am only 32. I play primarily jazz, so if i didn't get in with the guys who have been doing it their whole life, i'd be truly missing out on something. If they didn't accept playing with those younger and less experienced...well they just might not have a bass player.
Now to some, age DOES matter, as evidenced by the multitudes of craigslist ads that specify no one over the age of 30. They are free to do it, but it all comes down to the image of the band and the desire to hang out with people your own age. I don't think this way, but pretty much any original rock startup probably will(with founders in their early twenties). The thing to remember is that it has nothing to do with your ability to play the instrument, it's just for looks. Even at age 32, i am excluded from playing in a number of projects that want someone in their 20's. Normally these are groups playing music i am not interested in anyway, so i am not losing out on much.