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.
Out of necessity, I have been on various social media groups and the like for my city lately. It has not been good for my blood pressure.
First you've got the motorists who are screaming about ever having to slow down for anyone. This mindset drives me insane, at worst they're going to be a few minutes late. What's the big deal? Chill out, slow down and enjoy the cruise for a moment. They all rage against cyclists and pedestrians - the common talking point being "we pay for the roads through taxes on car use". Except they don't, taxes related to driving do not even come close to covering the costs where I'm from. You are being subsidized by non-drivers you entitled leaches.
Then we've got the landlords. I don't really get upset on the principle of rent-seeking like you guys do. Society isn't just and I've got a high tolerance for the morally dubious things we do for money in it. But listening to these people makes me scream. These parasites love bragging about pointless cruelty. I close my browser every time thinking Mao was too kind to landlords and shouldn't have stopped at China.
I'm politically apathetic, but if I stare too long into this abyss even I could be radicalized into the unabomber.