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'd take it immediately. Whether it rolls back time as well, or just my physical/biological age, having that reset is good. For time, it lets me be able to jump on financial trends before they happen such as bitcoin. Also lets me redo some stuff I fucked up. For biological age it's just regaining my youth and let's me get on HRT earlier than I did which would've been beneficial. I kinda threw away the past 15 years, so having a redo would be nice.