Young adult and adult, just because they have the most flexibility and options. The other stages have aspects I like about them, but the slow walking pace of elders gets tedious for me in game play. Teens have the annoying curfew, and they can't do things like travel to the future on their own, or use a car until they've been taught to drive. Children are even more limited, with some skills off limits. Toddlers are adorable, and I love getting cute screen shots of them, but after a normal life span's toddler stage has passed, I'm generally eager to age them up so I can do more with them.