You have a magic pill, which de-ages you by 20 years. You can take it once in your life only, so long as you are at least 20. What age — past, current or future — do you reckon is best to take it?
The day your health becomes a problem requiring more than regular effort to maintain.
That’ll rewind the clock to a lot of good years that maybe you can push back the decline a little further. Your clock will run out eventually, it’s inevitable. You just want to maximize the good years, not just youth or keeping yourself from death.