I'd agree - it's merely the risk-return equation that needs to be considered at a higher level once your over the "tween" period. There is absolutely nothing stopping you go forth and conquer - however, the probability that you have an inordinate number of external commitments [social and financial] are where the ability to be nimbal become more apparent.

Stopping work and having what ajg1977 states - "Mortgage, day care, insurance, kid commitments etc" - is the battle. Many successful startups have an average "age" of around 26 which seem to be [from some Y-Combinator article I read] the age when startups succeed for a whole raft of factors [age, martial status, flexibility, financial security, maturity etc etc] - there's always an exception to the rule however.

If you >30 - nothing's stopping you but yourself. The question often answers itself however - most >30 stop themselves due to externalities.

*but best of luck :P