I started doing stand up when I was 30 and often wish I'd started much earlier, but the truth is that I wasn't ready to do it before as I didn't have the life experience to give me things to talk about and I didn't have the self-confidence.
I'm not saying that there aren't good younger comics out there but the ones that I find most interesting tend to be 30+ and my favourites all seem to be in their 40s (Milton Jones, Tim Vine, Simon Evans, etc), but that, of course, could be because they share similar frames of reference to me.
Rhod Gilbert, who's doing fantastically well and really should've won the Perrier (or whatever you want to call it now) this year, didn't start until he was in his mid-thirties.
Personally I don't think you are ever too old, funny's funny, however being older does make it harder for you to make an impact as youth is worshipped by a lot of the people who commission TV programmes and promote comedy.