This is a direction from Allah to the mothers to suckle their infants through the complete term of suckling, which is two years. Hence, suckling after two years is not included in this address. Allah said:

(...who desire to complete the term of suckling,)

Therefore, the suckling that establishes Tahrim (prohibition, i.e., one cannot marry his mother or sister from suckling) is what occurs before the two years end. If the infant is suckled only after two years of age, then no Tahrim will be established. At-Tirmidhi under Chapter: `Suckling establishes Tahrim within the first two years,' reported that Umm Salamah narrated that Allah's Messenger said:

(Suckling establishes Tahrim if it is on the breast and before Fitam (before weaning, i.e., before the end of the first two years).)

At-Tirmidhi said, "This Hadith is Hasan Sahih. The majority of the people of knowledge among the Companions of Allah's Messenger and others acted upon this, that is that suckling establishes Tahrim (prohibition in marriage) before the end of the two years and that whatever occurs after that does not establish Tahrim". At-Tirmidhi is alone in recording this Hadith and the narrators in its chain meet the criteria of the Sahihayn. The Prophet's statement:

(On the breast) refers to the organ of suckling before the two years. Imam Ahmad reported a Hadith in which Al-Bara' bin `Azib narrated, "When Ibrahim, the Prophet's son, died, the Prophet said:

(My son has died on the breast and he has someone to suckle him in Paradise.)

Furthermore, Ad-Daraqutni related that Ibn `Abbas said that Allah's Messenger said:

(Suckling establishes Tahrim only within the (first) two years.)

Imam Malik reported this Hadith from Thawr bin Zayd who narrated that Ibn `Abbas related it to the Prophet. Ad-Darawardi reported this Hadith from Thawr who narrated it from `Ikrimah who narrated it from Ibn `Abbas. In this narration, which is more authentic, he added:

(Whatever occurs after the two years is not considered.)