All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The jurists of the four schools of Islamic law and Ahlus Sunnah Wal Jamaah are agreed that suckling after two whole years doesn`t establish the prohibition for marriage. They have based this on the following verse: "The mothers shall give such to their offspring for two whole years, if the father desires to complete the term. But he shall bear the cost of their food and clothing on equitable terms" {Al-Baqarah/233}. After the completion of two years, suckling doesn`t establish the prohibition for marriage. The evidence on this is that Umm Salama reported God’s Messenger as saying, “The only suckling which makes marriage unlawful is that which is taken from, the breast and enters the bowels, and is taken before the time of weaning.” {Transmitted by Tirmithi}. The majority of scholars from amongst the companions of the Prophet {Peace and blessings be upon him} and others are of the view that suckling within two years establishes the prohibition for marriage, but after that it doesn`t.
As for the Hadith that ' A'isha (Allah be pleased with her) said: "suckling the adult establishes the prohibition for marriage"it was based on the fact that the Prophet (PBUH) commanded Sahlah, the wife of Abu Hodaifah Bin Otbah, to suckle Salim (who is an ally) to become prohibited for her in marriage. This Hadith is the least problematic since most of the companions, who were acquainted with the context of the story and its events, understood it as a special concession for Salim because he was raised as their son and was treated as one. However, when adoption was prohibited and Salim became a stranger to that family, Allah has given him that concession. The wives of the Prophet (Peace and blessings be upon him), the honorable companions, and the scholars found that this is a sort of concession given by Allah's Messenger (PBUH) only for Salim. Later this ruling was abrogated. And Allah The Almighty Knows Best.