There's been a lot of good suggestions in this thread and others to assist the aging skateboarder with longevity. Different strokes for different blokes but here's what's worked for me. I am about to turn 46 and my body feels better now than it did in my 30s.
1) diet. obviously. know what works for you. this can take some time. I learned the hard way after 12 years that a particular diet had a detrimental impact on my health and mental health. I believe its different for everyone but generally speaking sugar and over processed foods are not helpful.
2) vary your activity. lots of people suggesting cycling. i do to. can't be skateboarding all the time... mix it up.
3) only skate when you're feeling it. no point forcing it. if a trick, spot or whatever isn't working out, move on... I no longer beat myself up trying the same shit 100 times. I find what's working that day and I do it to death. Sometimes, that just means 100 frontside slash grinds.
4) short frequent sessions. I used to have marathon sessions on the weekend. it just resulted in fatigue and pain for days after. I'm all about the 30 min to two hour sessions several times a week or more... Its really helped with my consistency (and dare I say progression), keeps me hungry and in less pain.
5) alcohol and any inflammatory foods do not help aging joints. absolutely do not help recovery time when injured. Limiting that shit has done wonders for my own health.
6) upgrade your insoles. I like remind medics but Vans popcush have treated me pretty well recently also.
Just sharing my own ideas and in no way offer the above as solve-all for all other old bastards.