Is the Ḥajj obligation fulfilled if one performs it before reaching maturity?
Q. If a child before reaching maturity has performed Ḥajj and then he matures… is it then necessary for him to do Ḥajj again?
A. The first Ḥajj does not fulfil the obligation; rather he must perform the Ḥajj a second time. This is because the first time he performed the Ḥajj it was considered voluntary, not obligatory. The Ḥajj of Islām is an obligatory Ḥajj, and so he must return for a second Ḥajj while the former is counted as voluntary.
Ibn ʿUthaymīn, Majmūʿ Fatāwā wa Rasāʾil (78/21)