What is the destiny of disbelievers' children who die at the ages of 14, 15 or 16? Will they be considered disbelievers or are they too young to be accountable for leaving Islam?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Scholars differ about the destiny of disbelievers' children who die before puberty.

However, any disbeliever's child who dies after the age of 15, according to the dominant opinion, or after the appearance of any of the signs of puberty, will have a destiny the same as that of the rest of the adult disbelievers. The Prophet, sallallaahu ‘alayhi wa sallam, said that any Jew or Christian who hears of him and does not believe in him will be among the inhabitants of Hell.  [Muslim and others]

Allaah Knows best.