Generally, girls and women did not have a paying job in 12th century England. They worked a lot, but rarely for wages. They helped with the family farm or fishing.
Boys were often trained to the work that their father did. The son of a farmer would become a farmer, the son of a cooper would become a cooper, etc.
The boys would learn the trade by working alongside their father and perhaps by lessons from others in the trade. The guild system with apprenticeships was not yet regulated and popular in this century.
The largest exception was priests, nuns and monks. Their families gave them to the Church as children or teens. They were then schooled and accepted into an Order.