You could argue that about loads though, couldn't you?
For example, children under 16 are not legally able to give consent to sexual activity. Lots of children under that age may say they are happy to participate in sexual activity, but the law does not seek them able to consent. Under 13, the law is very clear that regardless of the child's view, sexual activity is illegal. This is to protect children.
Children may say they're happy to work for money in shops, cafes and restaurants but the law says that they must not, except in v specific circumstances which are closely monitored.
Children may say they're happy not to go to school and to miss out on the opportunities that education provides. We still say there is a minimum standard of education all parents must provide.
Children are children, vulnerable because of their emotional, physical and cognitive immaturity. They do not have the ability of life experiences to enable them to make decisions that have long lasting consequences. We protect them for this reason.