Yes, since I have entered my 70s (I'll be 74 at the end of May), I can't help noticing the signs that it's downhill from here. The stairs of the world are steeper. It's an effort to bend far enough to put on my socks. I still buzz around town on my bicycle, but truly long trips may be a thing of the past. Like you I eat right (basically), exercise, and socialize, but I think you nailed it. Planned obsolescence. It only ever ends one way, as natural selection would have it.