The age thing is likely true, but the root cause is how much time you spend with your butt planted hard on the seat. This year I've been training harder with TRX and have less problems than in years past, even though I'm older. While I'm by no means a good stand up rider, I spend more time standing now and a lot of time light on the seat.
That said, I think suspension action has something to do with it as well. If your constantly getting smacked with the seat you might have an issue. This is my friend's problem, we are very close in speed so he is hard to pass in the AM. I just hang close, wait till his monkey butt kicks in and I'm gone.
