That's a tough one because all ages have their upsides and downsides. I loved how much fun I would let myself have when I was younger, but at the same time I cared too much about silly things or what people thought. I wasn't that worried about paying bills or retirement. I don't have the same worries or issues I did back then but I'm probably a lot less riskier and I do miss that fun.
I suppose, aside from the age I am now, I would choose something like 8 years old. Everything was fun, new, and interesting at that time. Nobody had the internet yet! Plus, my parents paid for everything.