So I take this pill, and I become physically younger. I don't move back in time, I'm still legally a 36 year old, but I look and feel like I'm 16.

It depends on how this works. Is the pill a magic spell where there's a poof and I'm in my previous body as it was 20 years ago, or is it just "damage and wear and tear are undone?" Because I've had a few surgeries I don't want to redo in the last 20 years; I don't want my wisdom teeth or appendix back. I've had a dental implant since then, does that reverse itself...is a bicuspid going to try to grow out of my skull through the titanium socket bone grafted into my face?

For practicality's sake I think no earlier than 43, simply because...at that point your younger self is a fully developed adult; if someone cards you and says "You're telling me you're 43 years old?" You can say "Yeah I've had some work done."

Much younger than 40 years old and you have to repeat portions of adolescence and/or childhood, which would be inconvenient at best.