We don't let kids buy guns, drink, drive and buy cigarettes for a reason.
Kids often make bad choices. You can't just "let them be responsible for their own actions".
Mentally disabled persons are often evaluated for competence and capacity and these assessments often guide what they are permitted to do. Even in regards to relationships and sex.
This is similar. Children obviously vary greatly and are at different stages of maturity. A method of evaluation is long past time. I'd expect such a thing to be more of an exception to the rule than the rule itself. An exemption, if you will.
ie: Child wants to be sexually active, maybe parents agree, or not.. so the child applies for a mental competency evaluation. It is very likely there would be a 30 day re-evaluation.
Just letting kids make their own choices is a resoundingly bad idea, particularly in an age where they can just vanish without a trace with ease.
By rule of thumb, most of them do not have full control of their faculties and this has a negative effect on decision making ability.
They do require protection, unfortunately this has not been revised in a very long time and may well be doing as much harm as good.
Children have rights, but they also have the need to be protected from their own bad decisions.