I was not going to comment, however I may as well.
Age is relative to how one feels and performs.
I'll be seventy in three months and I earn my living giving advanced flight instruction and also still fly unlimited aerobatics with no decernable degrading of my performance.
When the time comes that I have even the slightest concern for safety I hope I will be able to recognize the trend and I will then retire from commercial aviation, but if safe I will fly simple aircraft for pleasure if I so desire.
So in short it is not really difficult to determine ones performance as long as you are aware of what exactly to look for.
Chuck E.