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

                What is the ruling on making up missed
      prayers in the case of one who was negligent? Among us non-Arabs there are many who may
      pray one time, then not pray another time, until they reach the age of thirty or
      thereabouts, then after the age of thirty or forty, they start to pray regularly. Do those
      who have been negligent about their prayers have to make them up, and does the same apply
      to Ramadaan fasts?
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.