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?
I'll go for another angle, since the 'about 40' has been done to death.
If this is a pill that de-ages but does not de-mature you, go for it at 20. On the way to 20 you are maturing, but also aging. Who knows what mature, but with newborn level 'age' health might look like. Imagine the boost from a 40 year old to a 20 year old, but starting from a baseline of a 20 year old.
Sounds like a Black Mirror episode: The mind of a 20 year old in the body of a newborn. Incapable of doing all the things, that a 20 year old would think of. Many years until you are allowed again to do things, that you are actually mature enough to do.
Well, if it 'de-matures' then certainly would say older, at least mid 30s.
Lots of wiggle room in the question.
Some assume outright time travel, to repeat your life exactly as it were.
Then there's the question of maturation versus just the degenerative effects of aging.
Is it just the natural break down or is it also "wear and tear", and if "wear and tear", does that extend to damage from disease or injury? Can you reset to before an amputation if one were needed? If that's the case you'd want to hold on to that pill until you really start feeling things or have an injury. In school a girl was in an accident and got her leg amputated at 7 or so, so I wager for her the answer would have been certainly under 27 if it got her leg back, even if it meant dealing with a preteen body for a while.