In my case, I started training at age 46. I didn't have any illusions about being young in the martial arts to begin with. I was quite fat as well. However, by the time I hit my early 50s, I was feeling much better. I was never going to be competitive with those younger than me, but my body was doing what I asked of it.
Now I'm in my 60s. Things are just flat wearing out. My knees are full of arthritis. I have a high pain threshold, they don't hurt (doctor says they should), but they don't do what I tell them always either. My right shoulder is coming undone, punching air sometimes makes it pop out. My diabetes-caused neuropathy is making me walk funny, as my gait is messed up. I'm either foot-slapping or toe-dragging as I walk sometimes, and walking more than a block is murder.
My heart is angry with me, and afib is no joke. Standing up can make me feel like passing out. Doctor says it won't get better - terrific.
Still, I'll keep training somehow. Maybe it's time to start writing.