In the name of Allah, Most Compassionate, Most Merciful,
Let us first look at the translation of the Hadith mentioned in the Sahih of Imam Muslim and elsewhere:
Sayyida Aisha (Allah be pleased with her) reports that Sahla bint Suhayl came to the Messenger of Allah (Allah bless him & give him peace) and said: O Messenger of Allah! I see on the face of Abu Hudhayfa (signs of dislike) on the entering of Salim, who is an ally, (into our house). The Messenger of Allah (Allah bless him & give him peace) said: Suckle him. She said: How can I suckle him as he is a grown-up man? The Messenger of Allah (Allah bless him & give him peace) smiled and said: I already know that he is a young man.(Sahih Muslim, no: 1453)
Another version of this Hadith is as follows:
Sayyida Aisha (may Allah be pleased with her) reports that Salim, the freed-slave of Abu Hudhayfa, lived with him and his family in their house. She (i.e. the daughter of Suhayl and wife of Abu Hudhayfa) came to the Messenger of Allah (Allah bless him & give him peace) and said: Salim has attained (puberty) as men attain, and he has began to understand what they understand, and he enters our house freely, I, however, think that Abu Hudhayfa feels uncomfortable with this. The Messenger of Allah (Allah bless him & give him peace) said to her: Suckle him and you would become unlawful for him, and (the dislike) which Abu Hudhayfa feels in his heart will disappear.She returned and said: I have suckled him and what (was there) in the heart of Abu Hudhayfa has disappeared. (Sahih Muslim)
Before coming to any sort of conclusion, we need to first understand the background and context of this incident. Imam Abu al-Abbas al-Qurtubi (Allah have mercy on him) states in his commentary of Sahih Muslim:
Salim (in this Hadith) is Salim ibn Maqal ¦Abu Hudhayfa had adopted him in accordance with the customs of the Arabs. He (Salim) had been brought up and raised by Abu Hudhayfa and his wife as their own son. When the verse of the Quran Call them (your adopted children) by (the names of) their (real) fathers. (Surah al-Ahzab, V: 5) was revealed, the ruling of adopting children was abrogated (in that one can no longer consider an adopted son to be ones own). However, Salim continued to reside and enter the house of Sahla (the wife of Abu Hudhayfa) as he was a minor. When he grew old and came close to puberty, both Abu Hudhayfa and Sahla disliked the idea of him entering freely upon Sahla, but they found it difficult to mention this to him, given the fact that he had lived with them (and was brought up by them), hence they asked the Messenger of Allah (Allah bless him & give him peace) regarding this. The Messenger of Allah (Allah bless him & give him peace) said to Sahla: Suckle him and you would become unlawful for him, and (the dislike) which Abu Hudhayfa feels in his heart will disappear hence she suckled him and it so happened (i.e. the dislike of Abu Hudhayfa disappeared) (al-Mufhim lima Ashkal min talkhis Kitab Muslim, 4/186, Dar Ibn Kathir print)
The above clearly illustrates that Salim was adopted by Abu Hudhayfa and his wife Sahla. He had lived with them and was raised and brought up by them since childhood. However, Islam does not recognize legal adoption and an adopted child is not considered a real child, hence after reaching puberty, the rules of Hijab are applicable. It became very difficult for Salim to live with his adoptive mother due to this rule, thus the Messenger of Allah (Allah bless him & give him peace) advised Sahla to make Salim drink her breast-milk, whereby the rules of Hijab would be lifted.
It is narrated in the Tabaqat of Ibn Sad and elsewhere that Sahla would pour her breast-milk into a utensil each day for five continuous days and Salim would drink from it. He did not directly drink from the breast of Sahla, as it is not permitted to expose ones nakedness (awra) in front of a non-Mahram adult, let alone have him suckled. Thus, the objection raised by some non-Muslims that this was an immoral act has no significance, for Sahla did not directly suckle Salim, as he had reached puberty. Imam Ibn Hajar al-Asqalani (Allah have mercy on him) has also mentioned this in his al-Isaba fi tamiz al-Sahaba. (See: Tabaqat Ibn Sad, 8/271 & al-Isaba, 4/337)
This leaves us with the question whether is it allowed for an adult to drink breast-milk, and what implications would that hold?
It is a well-known and recognised fact amongst the majority of the Muslim jurists (fuqaha) that suckling is not permitted after two (or 2 and a half) years, neither does it affect the rules of Hijab and marriage. The Messenger of Allah (Allah bless him & give him peace) quite clearly mentioned this in one Hadith where he said: suckling is only valid if it takes place in the suckling period.(Sahih al-Bukhari, no. 2504 & Sahih Muslim, no. 1455) The Messenger of Allah (Allah bless him & give him peace) also said: Suckling (radhaa) does not prohibit (i.e. marriage) except which penetrates the intestines (m: meaning which serves as a nourishment for the child) from the breasts, and it is prior to weaning. (Recorded by Imam al-Tirmidhi in his Sunan, no. 1152)
Due to the above narrations, all four Sunni schools of Islamic law are in agreement that suckling and breastfeeding will only be considered (i.e. in effecting the rules of marriage and Hijab) if it takes place in the period designated for it, and it is of no significance after that period.
Thus, scholars mention that the permission given by the Messenger of Allah (Allah bless him & give him peace) for Sahla to give her milk to Salim was a special dispensation and an isolated case, and it cannot be generalized. Imam Abu al-Abbas al-Qurtubi (Allah have mercy on him) states that all of the Prophets wives with the exception of Aisha (Allah be pleased with them all) considered this to be a special dispensation, and this is the view taken by the majority of early (salaf) and late (khalaf) scholars. They considered the Hadith to be specific with Salim and Sahla, and are of the opinion that it is not permitted for an adult to drink breast-milk. If an adult did drink breast-milk, it will be of no consequence with regards to marriage and the rules of Hijab. (See: al-Mufhim, 4/186-187 & Ila al-Sunan, 11/119)
Moreover, Umm Salama, the wife of the Messenger of Allah (Allah bless him & give him peace) used to say: All the wives of the Messenger of Allah (Allah bless him & give him peace) rejected the idea that one can come to them (without observing the rules of Hijab) with this type of breastfeeding (i.e. the suckling of an adult), hence they said to Aisha (Allah be pleased with her): By Allah, we do not consider this but a dispensation given by the Messenger of Allah (Allah bless him & give him peace) for Salim. No one is to be allowed to enter (our houses) with this type of fosterage and we do not subscribe to this view.(Sahih Muslim, no: 1454)
Thus, in conclusion, it is not permitted for an adult to intentionally drink breast-milk; neither will it have any bearing on the rules of Hijab and marriage. The companion Salim (Allah be pleased with him) was given a special dispensation by the Messenger of Allah (Allah bless him & give him peace) to drink the milk of Sahla, the wife of Abu Hudhayfa, due to the special circumstances of their case. However, Salim did not directly drink the milk from the breast of Sahla, rather, she would pour the milk in a utensil and he would drink from it.
And Allah knows best