When you do the math, there's roughly 14 years between Kakashi and Naruto. At the end of Part I Naruto was 13 and Kakashi was 27. Tobi "Obito" showed up and unleashed Kurama the night Naruto was born. 16 years ago. But Obito died when he was 13. Assuming he's the same age as Kakashi, that was 17 years ago. I have no clue what happened to his body after he got crushed by that boulder, or if the person we saw in 599 today was actually Obito mentally, but when he fought Minato, no, he shouldn't have been an adult. He had to have become very enhanced, though, to go toe-to-toe with the Fourth like that, so maybe his body was enhanced then as well. I dunno. Who knows who helped him out with all that.