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.