Don't remember when I started. I think 2007. So I would have been 44. There were no step-by-step tutorials, nobody else training within 100 miles, so it was a lot of learning by error. Lots of minor injuries, and a few bigger ones. Twisted ankles mostly.
When kids started learning, I had a head start, so I got respect.
I had to be cautious, esp compared to the 14-20 year olds. Injury = lose work = lose money. My worst injury was indirectly parkour related. I biked to train with a visiting traceur. Biking home at night, I flipped over a guardrail, onto sharp rocks. I hitch-hiked, got a ride to ER. Plenty stitches and staples to put the R calf muscles back together. Infection. Swelling.
Where I live isn't great for "classic" parkour, so I saw way back that people here were either going to go for natural or tricking. I tried flips onto mats. Couldn't get it. So I got involved in Methode Naturelle, Hebertism, MN, Natural Method. Whatever you want to call it. Walk, run, jump, crawl, climb, balance, lift, carry, throw, swim, "combat"... it's a better match for me. The kids went tricking route.
I'll be 51 next month. I walk, run, bike, swim, play some beach volleyball, some lifting, minor climbing. I was in better shape when I was heavily training, but I got injured more. Parkour? Sure, I did a bunch of little vaults yesterday, and small wall pops. Nothing extreme, but I had a great time, lot of fun. I love the sense of freedom, of overcoming obstacles, problem solving.