Tosefta על נדה 1:2

A child may go on nursing for twenty-four months, and from that time forward, he is like one who nurses from a non-kosher animal--these are the words of Rabbi Eliezer. Rabbi Yehoshua says: a child may go on nursing even until he is a child of five years, but if he ceased and then returned (to nursing) after twenty-four months, behold he is like one who nurses from a non-kosher animal.