Granted, the Eleventh Doctor is the most childish of all his incarnations. But actually, he's most ancient as well. The big theme of Eleven's tenure is how he's an old man in a young body. He sees the universe through his companions' eyes, and it feels brand new to him again. Because both him and Ten are afraid of being adults, after what they did in the Time War. In the end though, Eleven grows up; he accepts his responsibilities on Trenzalore, and he becomes more adult. The Eleventh Doctor is one of the darkest; he's fighting a war, he's running from his past, and his future, and he feels his age more than any other Doctor. He can connect to children, but he doesn't feel like one. He's as ancient and forever as the others, but in this incarnation, Moffat's managed to use Matt Smith's verve to bring the show closer to kids. He hasn't ruined the character of the Doctor, it's just that incarnation that's more childish than others.
