My 20s were split between New England and CA, almost equally. Mid 70s - mid 80s.
Paris in the 1920s would have been nice. Not sure that any other particular time/place stands out. Maybe SF in the 1840s/50s.
I’d choose NYC in the 60s or Paris in the 20s - I’m leaning towards NYC. Either way, I’d want to be a white male - if I’m staying female, I wouldn’t go back at all. The modern world isn’t free of bigotry by a long way, but it’s progressed incredibly in the past few decades.
I spent my 20’s (1970-1980) in San Diego. Still can’t imagine a better place for it. (Actually, I spent my 0 thru 54’s there.)
I haven’t read most of the replies, but in my 20s I visited San Diego twice, and I also had a week in Spain with a couple of friends. San Diego is a wonderful city, but I gotta tell ya, that 5 days or so in the Costa Del Sol area of Spain is just burned into my brain.
If I was rich I would live forever on the beaches of Marbella and Malaga.
I spent most of my 20s in and around Boston in the 90s (where I met Shagnasty and his SDMB incidently). Being a 20-something guy working in tech in Boston during the dot-com era was pretty cool for the most part.
I moved in NYC in my late 20s, which I liked much better.
Vermont, New Hampshire, and the Virgin Islands
Had the most fun, and made the most money in the VI. Came back with large bank roll that set off the rest of the journey in a good way.