I am in the best shape I've ever been in at 38 but in terms of tree work, I peaked at 31. Both in terms of production and income. Making more than any other climber I've run across killed it for me so I move on to other challenges rather than get into the not climbing part of materials management, etc. Somewhere in there, I lost the love for the job and became a mercenary. It killed the job for me. If you're in it for the money, there are easier ways than trees....
Still, when I do tree work, I am as good as I ever was. The main difference is that I just don't GAS and inherently break fence boards and crush landscape plants. I just don't care about adding 2-3 hrs of rigging to a job to save potential damage to a $20-30 plant or board. I trend towards showing up, burying ground help, and then GTFO. My care and want to be zero impact aren't there. Now I approach a tree like a pit bull let off the chain or a bull in a China shop. I go up, tree comes down, I GTFO and let others sort it out. Bad attitude and why I do less and less tree work.