Is expiation (kaffarah) obligatory for someone who has missed many prayers, knowing that his age, according to the Hijri calendar, has not exceeded 15 years?

Missed prayers after reaching puberty must be made up, whether they were missed due to sleep or forgetfulness, according to the consensus of scholars, or intentionally, according to the majority of scholars, along with the necessity of repenting to Allah Almighty for intentionally abandoning prayer. Expiation is not required; rather, making up the prayers and repenting are. As for what was missed before puberty, it is not obligatory to make it up.