6 month age is usually quoted. This is a general rule and not to be taken very seriously.
For Breast fed babies - adequate weight gain and sleeping for 2-3 hours after a feed indicates that the milk supply is sufficient. In such cases weaning could be started later. Maybe even delayed by a month.
Occasionally weaning earlier than 6 months age maybe necessary when baby has poor weight gain and not satisfied after a feed and keeps asking for feeds frequently.