Is it permissible for a recently-matured 14-year-old who has experienced a wet dream to act as guardian for his sister in her marriage contract after the death of their father and grandfathers?

If a brother reaches puberty, he becomes eligible to act as the guardian (wali) for his sister's marriage. Indeed, some scholars have even gone further, asserting the validity of marriage performed by a discerning child, even if he has not reached puberty. It is narrated from Ahmad that if a child reaches ten years of age, he can arrange marriages and be married himself.