One of the most helpful things that I've done in my life has been to make sets of friends who are different ages than I am.
It helps me understand that although there are certain states of life people tend towards, aging isn't as harsh for everyone.
When I was 25, I thought that people who were 50 were ancient. But I play with a blues band and the drummer is 73 and still gets under houses to do plumbing.
My friends in their 80s who are still working are often slow... we all become geriatric at some point if we live long enough.
But I've also met people in their 50s who are really, really old. And I mostly date women in their 40s, and one of the most interesting things has been how old some people in their 40s are and how young some people in their 50s are.
A second thing that has helped me is that I keep playing with new stuff, just for fun. I keep taking up new instruments (I've been spending a lot of time playing banjo and piano, but this year I built a modular synthesizer and have been enjoying that way of making noise quite a bit). I keep learning new technical skills and because of the business I am in I can have my boss sell projects that allow me to practice the ones I think will be more profitable. And all that has taught me how much we can actually learn if we're just spending a 20-30 minutes a day on a specific practice, over the course of years.
So, I just turned 40 a couple of months ago, I sent my kid off to list first day as a high school senior. And I've got enough time to do about 3 more careers in my life. I've been a university professor, a semi-professional musician, and right now I'm a pretty good programmer and all-around IT worker.
Knowing all those people older than me, I'm able to see how much longer I might be here. I stopped drinking, got down to a very healthy weight, took up exercises that are fun (a lot of yoga, mountain biking, and hiking). I've gotten a lot picker about how I have romantic relationships. I travel more to visit my friends who have their kids tying them to a locale.
And knowing how much progress we can make by constant, slow study has made me super excited about learning general things. I read a lot of philosophy and history, but I also do a lot of playing with technology.
I started learning math again, because I feel like that will be important for understanding the various kinds of statistics I'll have to do to work with statistical tools like machine learning. CRUD apps have paid my bills for a long time, but I don't know how much longer I'll get a thrill out of building them.
So that's what I do. I'm not worried about making it big as a musician or a programmer because I like the process and I can get enough remuneration from it to keep everything going as long as I feel like living.
And I suspect that I am actually a lot quicker witted, can pay attention longer, and have a better memory now that I occasionally fast, am not drinking every day. I suspect that a lot of my friends in their 40s aren't "slower", they just have kids and are dealing with it by drinking, like I did in my 20s. And I do know that we age, but I also know that how we age isn't the same for everyone... with good luck, we have a whole lot of time to learn and play with interesting ideas.