Kids have no business in the industry no matter what anyone here thinks imo, and if the laws/policies were changed the industry and the society would have to adapt to it. That's ignoring that a lot of idols look way younger than they actually are anyway, so be it 20 and over or 18, the difference probably doesn't matter that much.
I also don't think that children should be training to debut from a young age, it makes much more sense for them to be in education or explore their hobbies through normal means, rather than being pushed into an environment of toxicity and unrealistic expectations. Healthy development is deeply important at these ages, and at the oldest I could only see a reason to give 16 year olds the opportunity to join an agency, though 18 would be preferable really.
Kids obviously learn quicker than adults, but if you have people who have pursued hobbies in dancing or singing they will be teachable, and will have had more experience than a child in learning dance moves or whatever else.
I could see companies giving some sort of support for children who have the skills to debut before say 16, and giving them ways to pursue their hobbies and skills, but beyond that ... nah.