I tried this last night. I collapsed into the floor at 17.
Where am I going wrong? I play sport, am in good shape, 12st, 6ft. Why can't I ever do more than about 25 of these?
Pushups are usually more difficult for taller guys with longer arms. The leverage works against you -- the movement is fundamentally a lever with its fulcrum on your toes and the weight on your hands. Ideal build for pushups is short, strong, light, with short arms.
I max out around 50, and I think it's hard for most to get beyond that unless you are either working out like crazy or you have a good natural build for them.