Speaking as a 36-year old in the throes of testing his app I bloody hope not! If you're sufficiently clear of responsibilities that you can do this, well, you'll always regret it if you don't.
I guess the reasons PG and co recommend youung-uns are mainly:
1) Energy (physical, I guess)
2) No serious financial obligations
3) No family obligations.
I think #2 is probably the biggest. However, if you are older and have no debt/mortgage, plenty of everything, they why not. #3 you can't do anything about really, but as long as you don't mess with #2, the #3's in your house won't be too upset.
Also having kids teaches you to sleep efficiently and economically (you sleep when you can).
I find it fascinating to watch startups, I think its an awesome thing to do.
