If a person starts to pray regularly after the age of forty, what should he do about the prayers he has missed?

Praise be to Allah.

The person who is in this situation is a kaafir guilty of kufr akbar (i.e., he is beyond the pale of Islam), according to the more correct of the two scholarly opinions, so long as he does not deny that the prayer is obligatory. If he does deny this, then he is a kaafir according to the consensus of all scholars. If he repents and starts to pray the obligatory prayers and to fast in Ramadaan, and he continues to do so, then he is considered to be a Muslim, and the prayers and fasts that he deliberately failed to do in the past do not have to be made up, because the Prophet (peace and blessings of Allaah be upon him) said: “Islam wipes out what came before and repentance wipes out what came before”, and because the Sahaabah (may Allaah be pleased with them), when they fought the apostates at the time of [Abu Bakr] al-Siddeeq (may Allaah be pleased with him), they did not tell those who came back to Islam to make up the fasts or prayers, and they [the Sahaabah] are the most knowledgeable of the sharee’ah of Allaah after the Messengers, peace and blessings of Allaah be upon them.