I am forty years old and can easily do a dozen handstand pushups (BTW, handstand pushups against a wall is the absolute best upper body exercise for overall shoulder strength; not to mention that it also hits your triceps, upper back, and core). I also do dips, pullups, and hindu squats. All four bodyweight exercises are performed once a week in a high intensity 45 minute workout. I also run hard a couple of times a week in 10 minute bursts.
But. That is not enough to compensate for my sedentary work as I have learned recently. I need to diversify my physical activities as this article cogently demonstrates:
Disclaimer: I have the conviction that if someone performs more than one high intensity workout a week it is overtraining ("high intensity" properly defined by its original definition, e.g. Mike Mentzer). The point I am making is that the linked article above contends that we should be performing regular moving-activities throughout the day to break up the sedentary lifestyle that many of us bookworms possess.