What must one do who has missed some prayers, or delayed some, or lacked devotion in some, or did not pray Fajr until Dhuhr?

If the negligence occurred in childhood before puberty, then there is nothing upon the person because they were not held accountable at that time, as the Prophet, peace and blessings be upon him, said: "The pen has been lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane until he regains his sanity."

However, if it occurred after puberty, then repentance is obligatory, and the missed prayers must be made up. If their number is unknown, then one should make up what is most likely to clear one's conscience. Neglecting devotion (khushu') diminishes the reward of the prayer but does not invalidate it.