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?
The problem with that is that (now that I'm in my 40s) I don't like to be around kids for long periods of time, and have come to see anyone under ~25 as a kid. Would I be able to stand being around myself (or my new peers)?
It would be weird to have kids with someone who is effectively 20 years younger than you, though.
You either go with someone physically the same age who is of a completely different generation from you, or you go with someone mentally your age who you may not be able to have kids with without risking a higher chance of congenital disorders.