No age restriction but that's why a health practitioner would need to give consent to what they felt was safe and acceptable.

So if young consent might be given for a non-sexual relationship by parents/guardian and health practitioner. The health practitioner could discuss why it's important that there's no sexual aspect, the health reasons while making sure that both agree and understand that. Again it's about protecting because imagineray lines always risk removing protection for someone who dips over a line and risks forcing relationships underground.

It's like being homosexual. If it was illegal and socially unacceptable would people stop practicing gay sex or would they be secretive and indulge in dangerous sexual practices? Making it legal means people have relationships more openly, friends and family know and can be supportive, health practisioners can offer health advice and support and generally people are safer.

The objective is to make people safer. That should be the acid test. At every stage ask the question "are people inlcuding children safer and less likely to be physically or emotionally harmed?". If not then ditch it. If it looks too difficult to police and apply then stick with the current age test but I'm suggesting perhaps there's scope for research into a better, safer system. We shouldn't be afraid to research and learn.