One phrase I associate with startups is "dumb enough to try". Startups usually have to break rules, and the most reliable way to pull that off is to not know you're breaking rules (i.e. youth).
It's not a death sentence for older entrepreneurs, though. A simple countermeasure is to live/act/feel young.
Presumably a great strategy is to work for your 20s in a certain field (say, IT/web), and then in your 30s spend a year or two working in something tangential (say, trucking/logistics), and then do a startup which combines the two. Then you're sort of a newcomer to one, but with a lot of experience to bring to bear, as well as general workplace skills.
I know that I've seen the opposite behavior. When someone is young and unknowledgeable, they often are easily intimidated by "experts" and what the experts say is the correct way to do things.
If you're experienced, you know the conventional wisdom well enough that you can set it aside when you determine it isn't appropriate.
It's not a death sentence for older entrepreneurs, though. A simple countermeasure is to live/act/feel young.