Mujahid said that Sayyiduna Ibn Mas'ud (may Allah be pleased with him) was asked about the wealth of orphans, and he said: "When they reach maturity, inform them how much zakah has become due on it; then if they wish they may pay it, and if they wish they may leave it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9557