Naming 4 ppl that suit your point are not grounds for being right. I can name 4 that go against what youre claiming..Kakashi, during the first half of Naruto he was strong, but look at how much stronger he is now. Unlocked MS? He is way older than 16. Kabuto..older than 16. Look how he has improved. Madara didnt even unlock rinnegan till right before he died, he got stronger after he was 16. And even Orochimaru. He went off and invented tons on new jutsu after he was much older, nearly perfecting Edo Tensei.
"unless they have something special about them"?? Having something special about you is the name of the game when it comes to being a prodigy. Naruto is only 15-16 and continues to get stronger. Naruto HAS something special about him, in fact he is the most special in the series, that is what makes him the number one prodigy. He's not done growing, he is a teenager and is currently one of the strongest...and im sure hes just going to continue to get better...Prodigy.
And Kankuro? Kankuro just aquired whats arguably his strongest puppet in Sasori.