When to Wean
Your mother in law, your neighbor, your best friend and anyone else you ask (or don't ask) have different answers to the question, "When to wean?" Let's see what some of the "experts" have to say.
The Health Authorities
The American Academy Of Pediatrics recommends exclusive breastfeeding for the first 6 months and to continue with partial breastfeeding, that is, with complementary food, for at least a year. The World Health Organization also recommends 6 months exclusive breastfeeding but they recommend continuing to nurse until the baby is at least 2 years old. Those are the minimums. Both the AAP and the WHO go on to say that breastfeeding should continue on past the prescribed minimum time "for as long as mutually desired". Likewise the AAFP states, " breastfeeding beyond the first year offers considerable benefits to both mother and child, and should continue as long as mutually desired." None of the above groups gives an upper limit.
The Anthropological View
Kathleen Detwyller, Associate Professor of Anthropology at Texas A&M University, looks at this question from a purely biological point view and compares humans to non-human primates. Many studies have been done on animals in an attempt to understand their criteria for weaning. Dr. Detwyller applied these same criteria to humans to see when humans would wean if we lived completely according to our mammalian instincts without any cultural or similar restraints. A widely accepted idea is that large mammals begin weaning their offspring when they have quadrupled their birth weight. In Western industrialized society, humans quadruple their birth weight between 2 and 3 years of age. Other data shows weaning as being when the offspring reaches one third of his adult body weight. In developed countries, that would be 6 years old for a girl and 7 for a boy. Another view considers the eruption of the first permanent molar as the time for complete weaning. This occurs in humans at age 6. Six years of age is also the time when a child's immune system reaches full maturity. Until then, a child's immune system needs the active immunities in breast milk to fully function.
Prof. Dettwyler concludes that human offspring would naturally wean sometime between the age of 2.5 and 7 years.
Breastfeeding does not happen in a vacuum. The culture we belong to has considerable influence on our breastfeeding practices and when we wean. Western society is slowly turning back to a breastfeeding culture. Mothers in traditional societies, i.e. non-European, non-US, tend to wean around 4 years old. Mothers in the US wean by a year. Within modern societies there are sub cultures that adhere to child led weaning, such as La Leche League. Breastfeeding specialist, Dr. Ruth Lawrence, states that "weaning is rarely child initiated until age 4." It may even be much later.