Two of the most productive, and accurate, and quick -- while accurate -- fellows I worked with were getting into their 40's. Our boss was a bit younger, but stood well apart from his peers in capability (and pleasantness).

My impression from this: It's the person, not the age.

EDIT: I've had the opportunity to work with some plenty smart women, too. (I realized the prevalence of the masculine in the above.) Again, several were "older". Same conclusion.