I'm in my 30's and I feel I have one huge advantage over numerous 20-somethings: ability to do short term contract work, and fund the startup that way. This is assuming :

a) you have enough networking/experience to land sufficient paid contract work fairly easily b) your short term contract (3-6 months) can provide you just enough income to keep you going for the rest of the year.

Sure, I certainly don't have the careless energy level as I had in my 20's, and have a lot more responsibility, but the ability to work smarter, having a better eye for what may or may not work through experience, and identifying the right people to hang out with surely makes up for the shortcomings.

Last, but this is huge for me personally, is the feeling of 'this is my last chance' that's ever more so prevalent now than in my 20's, where I always knew I had some leeway years left as an excuse to not put in 100%.