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?
Key question here. I'd take it at 37 and go back to being 17 with the skills, knowledge, and experiences and most importantly income of my 37 year-old self.
But, I'd pass myself off as 18. Unless, of course, it's not a secret. In which case the strategy totally changes.
If it's known and knowable that I took this drug, then I'd take it at 55 and de-age to 35. Then, when my kids are in their teens and tweens, I'll have the energy for their B.S. Also, when I retire at 95 (b/c seriously, retirement wont be a thing for me), I'll only be 75 and I'll still be able to fight off some of the horde of lawyer-bots, advertisclones, and chain letters that are coming after my pension.