I'm in your ballpark age-wise.
Physically I'm definitely not as capable as I used to be. Doing construction work like I did when I was 20ish would be very rough. I'm still holding up in terms of slower activity like wilderness backpacking, but strength isn't there, and there are aches and pains.
Mentally I'm probably the best I've been in my adult life, but there was a decade or so when that wasn't true. I started slipping, from laziness really, and it took conscious effort on my part to reverse that. My personal theory, founded on nothing but my own observations and biases, is that a lot of middle-age mental decline comes from losing the desire to make the effort. Older age will likely be different, but that's how it feels now.
Re your last point, I recently saw someone who was the spitting image of me 35 years ago. It was surprisingly unsettling.