It's true! At least for me. I'm in great shape. I'm as strong as I've ever been. I'm an obligate exerciser. But...I pull my muscles more often, I get sore and stay sore longer. Should I get out of shape (unlikely) it takes longer to get back into shape. I'm crazy about skiing and ski as much as I can. But the first day of skiing of the season really makes me sore, more sore than I used to get, despite the ease and benefits of shaped skis (I learned on straight skis, which were much more difficult to use than the newer designs of today). I'm more apt to get altitude sickness, and my balance seems to be getting worse, too. When I was young I used to run up and down mountains with no worry of falling in either direction. Walking on ice was no problem. Yet, just this past winter I bought special treads that fit over one's shoes to provide greater traction on ice. This need never would have occurred to me five or ten years ago.
And, last December I got "tennis elbow" by scraping ice on my car's windshield, of all things! It still hurts despite twice weekly sessions with a physical therapist (he's great!). When you're young, you can afford to be out of shape, because it only takes a week or two of exercise to get back into shape. When you're older, it's much more important to work out consistently, if not compulsively, because the negative consequences of being out of shape are greater. I don't feel old, despite what my kids always tell me. Even though I'm in good shape, reasonably strong and muscular, and thin, I developed sleep apnea two years ago and have to sleep with a pump (CPAP) that keeps my pharynx and trachea inflated so I don't stop breathing during my sleep. When something like this happens, it's a bit of a surprise (and a bit scary), because I don't think of myself as old and less functional. But, it is what it is. You adapt, and you're glad that you don't have something worse, because there are always worse things to have. Believe me, looking at me, you'd never know these things. You'd think I was perfectly normal and healthy, and maybe a bit distinguished. But, normal and healthy is an age related norm.
When you're young, you don't often think that life is finite. But, it is. And, this becomes evident as you get on in life. Now, at the age of 59, my attitude about things has changed. For example, I'm much more directed in my reading and learning. I don't feel that I have the time left to dissipate in undefined, inchoate interests. I really am more goal oriented, and these goals are more realistic and defined according to a shorter time line.
I've been married to my wife, a prominent academic physician researcher (and INFJ) for 35 years, so we know how to live healthily and have done so throughout our marriage. Yet, at the same time that I feel experienced and secure emotionally, I feel more vulnerable, physically. One goes up and the other goes down. Ironic, it is.