Going with the just de-age interpretation and not time travel, it has to be late enough I could still pass for an adult but I’d want it before any of my chronic health conditions emerge so I can mitigate them. I don’t want to look younger, I just want the health benefits.
I can’t go back to being a kid because where the hell would new identity documents come from? I still have to be able to live my current life more or less. I suppose 35 is the absolute minimum for me to take it, at 15 I wasn’t getting carded buying alcohol. I reckon at that age with the right presentation I could pass for 20 at least, and a 35 year old seeming that young isn’t completely unheard of.
I can’t go too much older because issues start compounding in my 20s. I’d love to have picked a post development age - aside from my health, I didn’t really get comfortable in my own skin until then - but it’d be too late. Maybe 40 so the worst of puberty is over, but that’s probably my limit.