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.