It is just an arbitrary standard invented by your Mirza...
It is just an arbitrary standard invented by your Mirza Ghulam Ahmad Qadiani who wrote that: "Since the beginning of the world there never was any example of even a single person who got, '23 years like our chief the Holy Prophet (S) and who was wrong in his claim of getting revelation from God…. If you find a person, who claims, to be sent by God and if it is proved that 23 years had passed since the claim of receiving the revelation from God..........
then you should believe that he is from God". This self-invented standard of the truth or falsity of the claim of prophethood is very amusing. Let us suppose, a man heard our Holy Prophet (S) proclaiming his prophethood in the beginning and said that he would wait 23 years to see whether Muhammad survived that period or not, do you think he would have been excused and pardoned by God? And what if he himself died during the life-time of the Holy Prophet without accepting Islam? What a rubbish!
And remember that many true prophets had been 'killed within one, two or three years of their prophethood. What would have been the position if someone, during the prophethood of Prophet Yahya (John), refused to believe in him, saying that as he did not live 23 years he was, God forbid, a liar". And also some impostors have lived more than 23 years after their claims. Why make a condition from Adam' up to the Holy Prophet"?
If it is a Divine criterion, It would remain true ever after the Holy Prophet of Islam. In fact, "after the Holy Prophet" would have been more appropriate, and a better period for checking, because now we know that no other religion is to come from God. Still we see that many religions having no connection with Islam have appeared after the Holy Prophet and have prospered. For example, Sikh and Bahai religions.
They themselves do not claim any affinity towards Islam and still they have prospered though the Muslims and Qadianis both agree that these religions are wrong. The Church of Jesus Christ of Later Day Saints (Commonly known as Mormon Church) was founded by Joseph Smith in 1820 A.D., when he claimed to have received divine call for prophecy in the name of the Most High. He claimed to receive divine revelation written upon golden plates which he was able to translate.
The first edition of the book of his revelation was printed in 1830. His associate, Oliver Cowdery, also claimed to be ordained by angelic visitants.