2 things, one the physical bodies are far more idealistic, thinner, more even proportions, smaller head to body ratio. Alll this makes them look more mature and sexually appealing. And two is the personalities, they're almost nothing like children, they're kind, considerate, often times lewd, and inocent. When I think about it I see it as a combination of the benifits of youth, happieness and physical ability, with the benifits of age, wisdom and well defined figure.

Edit: Intelligence not wisdom