We can derive a more objective similar concept from the principle of informed consent. If someone isn't intellectually mature enough to understand what they're consenting to, then it is not valid informed consent. Some people are smart when they're younger and some people are stupid their whole lives, so validity of consent depends on the individual. This could be called 'maturity of consent' or something to that effect. The concept of 'age of consent' is dependent upon an arbitrary number, so this is just more arbitrary illegal bullshit imposed by the State, in violation of the consent of everyone affected by it.