The anterior deltoid is also recruited during push-ups, especially when your trunk and legs are in a decline position.
This business of age and gender being the determinant of how many push-ups one should be able to do is nonsense. (Even if it came from ACE, from which I hold a primary certification.) I'm 46, and I normally can do between 40 and 50 straight-leg push-ups with my feet on an 8" or 12" elevation. And there is another Cathe fan (who, alas, no longer posts on this forum) who is in her 50's and within the past couple of years met her goal of belting off 100 "guy" push-ups.
A-Jock