(This started out as a comment on JeffE's answer, but became too long, and I don't want to clutter his post.)
This is a gross simplification, but what hiring committees care about is your future research potential. In the absence of a crystal ball, they have to extrapolate from your previous achievements: The more in less time, the better. For this, your actual age is irrelevant - it is only important how long you have been an active independent researcher, which usually starts after your PhD. (This also touches on the point @user11192 made.)
Tl;dr: It's not age, but age minus time of PhD, divided by number of publications (and grant money squared).