Answer
(Fatwa: 675/445/L=1431)
Almighty Allah has created too many signs in His creatures that if a man ponders over it he can recognize Almighty Allah. If man reflects inside himself he can come to know Almighty Allah. And when a man is in this world and specially where Muslims are present and the message of Islam reaches him, then accepting Islam is obligatory on him. Had he been in such a deserted place where he could not receive the message of Islam then too he had to believe in Allah in order to get salvation. The age of fifteen is enough to learn the religion of Islam and to know Almighty Allah; hence if your friend died at the age of fifteen he died in the state of kufr.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband