I'm 42 and saw that too but did not read the study linked in the article. I'm in the best shape of my life, not bad for a totally sedentary job and two young kids. When I look around at most other parents, I feel like I am in the top 10% or better fitness wise.
Unfortunately, I don't like the implication that one has to be a "triathlete" to be healthy or have quads of steel at 74. I believe Mark would roundly criticize this type of endeavor although I suppose with a base of extreme fitness it is possible although as Mark points out this type of training was not in the long run beneficial for him. However for those of us with demanding jobs, kids, marriages, and yards/homes to take care of (I am both the gardener and housekeeper) spending time training for a triathalon is an incredibly selfish, indulgent type of behavior. Given a choice between spending time with my kids on the weekend or training for a tri, I would pick the former any day.
One important point of the PB is that you can be extremely fit with short intense bursts of heavy lifting and sprinting not running marathons, swimming endless laps, or biking until you puke. Screw training for a tri, I think all of us can have legs like the guy in the photo without the type of lifestyle that mandates an unrealistic training schedule for most of us.