We have sent this question to the offices of the following grand jurists and the answers which they have sent us are the following:

Office of Grand Ayatollah Khamenei (may Allah grant him long life):

There is no problem in it per se.

Office of Grand Ayatollah Makarem Shirazi (may Allah grant him long life):

There is no problem in it as per the Islamic law.

Office of Grand Ayatollah Sistani (may Allah grant him long life):

It is not permissible, as an obligatory precaution.

Office of Grand Ayatollah Saafi (may Allah grant him long life):

There is no problem in it.

Also, read the following answer given by Ayatollah Mahdi Hadavi Tehrani (may Allah grant him long life): 

A woman’s milk is not among the prohibited things and if babies are said not to be breastfed beyond two years, it is because of respecting the mother and child’s rights, not because it is prohibited to breastfeed after the second year. Therefore, there is no problem in the given case.