I would tell anyone that thinks they can't do it that in boot camp we did sissy pushups on our knees to relax after getting smoked doing the regular ones. If someone can't do regular they can get on their knees and do 20 * 5 times or any small rep multiple times. Set a goal. Make it easy and then with the habit developed open up the challenge door.

Note your handicap. Get down and do it. Eventually you lose the knees and you start doing stiff board pushups.

It's all about being honest with yourself and pushing your envelope. The best thing about pushups is as you gain muscle mass you get heavier and you gain more muscle mass.