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?
24 seems a decent time. Returning to the brain plasticity of a 4 year old with all of the memories and knowledge of a 24 year old would produce a horrific intellect and potential. Society would leap forward in any task you decide to advance. The indescribable isolation may well be the only issue, but piles of money heal many forms of hurt. See: Magic Johnson