A. `A'isha said that the Prophet, blessings and peace upon him, gave a special permission to Abu Hudhayfa's wife when she asked a similar question concerning her adopted son Salim. He was past puberty. He said to her: "Breastfeed him so that he may enter in your presence while you are uncovered." Narrated by Muslim and Imam Ahmad. However, this was a special permission to that person only. Umm Salama said that none of the Prophet's wives considered that this permission was applicable to them: narrated by Muslim, Ahmad, Nasa'i, and Ibn Majah.
The Companions also understood, like the Mothers of the Believers all except `A'isha, that any breastfeeding (rida`) that is done after age two does not make the suckling a mahram. Ibn `Umar said:
"Allah said: "Mothers shall suckle their children for two complete years, for those who wish to complete the suckling." (2:233) Therefore we do not consider the suckling that takes place after two years as a cause [for mahram]." Ibn Qudama, al-Mughni (7:742).
Imam Abu Hanifa, Malik, Shafi`i, and Ahmad as well as Sufyan al-Thawri all stipulated that the limit for breastfeeding which results in mahram relationship, is two years.
The quantity needed varies from a single drop to five sessions. The safest view is the latter and is the view of Shafi`i and Ahmad. The definition of "session" is whatever average quantity a mother knows from custom or experience will sate her baby so that he is no longer hungry.
The milk must reach the child's belly through the mouth, but it is not a condition that it suckle directly from the breast. He can drink the mother's milk from any container such as a bottle, if she so desires. And Allah Almighty knows best.