The following are the responses of the grand maraji':
His eminence, Ayatullah Khamene'i (may Allah prolong his life):
In the case of the question, it is not permissible.
His eminence, Ayatullah Sistani (may Allah prolong his life):
It is not valid as per obligatory precaution, even if the leader is 10 years of age.
His eminence, Ayatullah Saafi Golpaygani (may Allah prolong his life):
The leader of prayers must be baligh.
Ayatullah Mahdi Hadavi Tehrani (may Allah extend his blessings):
In this case, one who has better recitation or at least, correct recitation and is more religious is more suitable to lead the prayer; in any case, jamaa'ah prayer is something good.
 Inquiries from the offices of the grand maraji' (may Allah prolong their lives) by IslamQuest.