WHO recommends two years or longer.
Answers to frequently asked questions on breastfeeding for healthcare providers and families.
www.cdc.gov
Humans evolved to breastfeed for 5-6 years, but some of our ancestors had more reproductive success weaning earlier because breastfeeding suppresses ovulation in some women. Don't expect breastfeeding to be 100% effective birth control, though. Use condom.
Why wean at one year? It's a cultural choice. In some cultures, breastfeeding to 5 years is completely normal.
Refrigeration and other technologies have allowed us to use the milk of non-humans to sustain our children
Children will typically wean themselves between 2-7 years. Personally, I've found that breastfeeding is the best way to keep the kid still when I'm reading to him. Unlike a pacifier, breastfeeding helps align the teeth and reduces the chance that the kid will need braces later. Breastfeeding, like a pacifier, provides comfort, and comfort is essential to brain development. There are lots of ways to comfort a child, but breastfeeding is a good biological default. I've been keeping track of my kid's brain development with the CDC's milestones list, and the only milestones on the 5-year list I still have to look forward to are the movement/physical development milestones.
Developmental milestones are things most children can do by a certain age
www.cdc.gov