I do not know of a recommended age for weaning. There are many factors involved, so one size can't fit all. The American Academy of Pediatrics currently (2005) recommends: "Pediatricians and parents should be aware that exclusive breastfeeding is sufficient to support optimal growth and development for approximately the first 6 months of life and provides continuing protection against diarrhea and respiratory tract infection. Breastfeeding should be continued for at least the first year of life and beyond for as long as mutually desired by mother and child."

The tradition cited by Rashi is that Isaac, the first baby to be born Jewish, was weaned at 24 months, and that seems to have been the convention through the ages. Nevertheless, according to Jewish law, a healthy child can nurse up to age four, and a sickly child through age five. However, a child older than 24 months who has stopped nursing for 72 hours cannot resume, unless there are health issues involved. Anyone older than this is forbidden to drink directly from the breast; however, expressed milk is permitted. (Yoreh De’ah, 81:7)

If you are having trouble with nursing or deciding when to wean your child, you might want to consult your local La Leche League people.

Here's a related Q&A: Is human breast milk dairy?.

Yours truly,

Rochel Chein