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?
In that case I sell it some billionaire for a 100 million. It is quality not quantity of life. It isn't like I am losing twenty years of life I am gaining a 100 million dollars.
I find it funny that no one else (including me) even considered selling it!
Now feel free to answer if you can't :)
Edit: although I bet you could fetch WAY more than 100 mil for it. The oldest and richest people in the world would probably give up the majority of their fortune, especially if they were close to death. Auction it off!