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?
Yeah stick with 40. What are you gonna do, be like "yeah it feels great being only 50 years again! Glad I passed up having a second twenties." If you heard someone say that you'd think they were insane.