10ish was a great age. Old enough to not need adults around 24-7, but young enough to have an imagination and no cares in the world. There isn't much in this world that can compare with gathering all the kids in the neighborhood and playing walkie-talkie hide-and-seek across the whole town.
I wouldn't mind going back to that age.
The beauty of it, too, is that it's a young enough age to not be jaded about life, so I might not even notice the fact that I was stuck there forever.
22 was also a fantastic age, for me.
Fresh out of college, still in great shape, able to stay up all night partying and be great the next day, a whole lot of girls, had a brand new job which hadn't become annoying yet, lived in an apartment with hardly any concerns, etc.
That wouldn't be a bad spot either, though I'd eventually get tired of being stuck there, I think.
Any age beyond that would probably end in pessimism. How could you not tire of being stuck at one age, unless you were blissfully unaware?