I don't think 'cool' suddenly stops. I think it decays exponentially, that is to say it has a half-life. So, firstly, this means that, provided there was some there to begin with, there will always be a little bit left. Secondly, and more importantly, given that the bass player is always the coolest member of the band, bass players remain cooler for longer. Finally, this means that in any band they can be almost twice as old as the next oldest member and still be the coolest member of the band. OK, I made that bit up, but you get the picture.
YMMV