Mujahid said that Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: "The guardian of an orphan should keep count of the years (of his age); then when he reports that he has reached puberty, he should tell him: You owe zakah for such-and-such number of years."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9558