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I find that even at 20 it's very easy for me to cause joint issues after several years of inactivity. If you do too much too fast it'll cause problems no matter your age - although I'm sure age is something that you should take into consideration.

If you're concerned, there are alternatives like swimming and biking that are lower impact.

A good alternative is tabata [1] sessions on the rower. 20 seconds as hard as you can 10 seconds very gentle (just to keep the timer going). Do it 8 times for a total of 4 minutes - it starts to get pretty grim around the 4th sprint. It's a great finisher or warm up :-)

[1] http://tabatatraining.org/

I strongly recommend rowing. Full body, low impact, and you can do it in your home, which lowers the activation cost a lot.

You can do HIIT in many low-impact sports. Swimming comes to mind, but there are plenty of great high intensity sports which work with agin bodies.

Most coastal areas probably have outrigger paddling clubs or dragon boats or crew. I can't recommend outrigger canoe paddling enough.