Back to the 50's or 60's when the everything seemed to be a "Leave it to Beaver" kind of world. At least on TV and in the movies, people seemed to be kinder, more neighborly, less stressed, and really could leave their doors unlocked. It's probably a romanticised view, but it seems like it would be nice.
As for what age I'd stay at, I'm really not sure. I'd probably stay at 23 or so, before some medical issues cropped up and I was still a healthy hottie. :-)