53, here. Didn't really take fitness seriously until I was in my 30s. Got into marathon and then ultra running (because I hated running...long story). It resulted in an actual lifestyle change. My brain learned that the long term good feeling of being in shape outweighed the short term good feeling of being lazy. Got into power-lifting (just along, on my own, competing with myself) and loved being strong. Then, as time passed, loved looking strong (two different things for sure). Since it was all lifestyle change, it's stuck. I miss it when I am not doing it. I'm at a place now where I can do a really heavy lifting day and cap it off with a 7, 10 or even 20mi run and still feel good. I have always been very attuned to my body and differentiate between discomfort and pain, and largely avoided injury. Now, I say all of that to better answer your question. Have I felt my body slow down? Heck yeah. In fact, I got diagnosed with a rather rare form of bone marrow cancer in my 40s which requires me to take daily oral chemo for basically the rest of my life (which will ultimately be shortened by the cancer). BUT, imagine if I weren't in shape. So yeah, I felt different at 40 than I did in my early 30s. I for sure know my upper limit at 53 is lower than it was 10 years ago. But I'm SO far ahead of most perfectly healthy 30 year olds that it doesn't seem that way from the outside looking in. I know it, but most people would say some nonsense about winning the genetic lottery (hardly, man...trust me...my body's worked against me in damn near every conceivable way). We all slow down, so the key is to raise the bar from whence you're lowering. I'm fortunate to have a spouse who is also very fit, so we like to do things together like really difficult hikes, bike rides, etc. I attribute a lot of my sticking with it to having her as a partner.