Strikes me as completely self-centered and narcissistic. Like he's growing older but not growing up.

As someone nearly 60, a lot of this rings true. Especially

One reason that the experience of growing old can feel jagged and abrupt is that there is a disconnect between how old we feel and how old we are. You often hear people say “inside I still feel young”.

Yes my "inner self" is still the same as it was when I was 25, at least it seems that way. I have a lot more experience of course but I still feel like the same person I was then.

When I was a young person I imagined that being 60 would feel fundamentally different. But it doesn't. And fortunately about ten years ago I began to work on keeping myself in shape (something I did not focus on as a young person) so I'm actually stronger and fitter than I was then and still in overall good health. Fingers crossed.

I can't relate to this at all. I never felt conscious about how old I was until I was in my late 60s, when it dawned on me I was old. Up to that point it never entered my head. Never thought of myself as young (when I was) either. Perhaps some people are more conscious.