There is no basis for the notion that children are "not developmentally mature enough to make informed decisions about sexual activity". Sex is not difficult to understand, and its "consequences" are primarily cultural which could change with changing culture.

The fact some notion is "generally accepted" does not necessarily mean it objectively correct. The age of consent is arbitrary and varies between different countries.

If a child "appears" to give consent, then the child is most likely actually giving consent, assuming absence of coercion or manipulation.