The Major Jrs makes hockey very different when it comes to drafting young guys. It’s basically minor league hockey, but high school kids play in it, they get a pretty brutal schedule and still have to go to school. In a lot of ways it’s college basketball for hockey.
A league has a right to say that players of a certain age lack the physical and mental maturity they desire to project the image they wish. There was a serious drop in the quality of NBA play and an uptick in off court legal issues when they were drafting high school kids. I’m not sure play quality has gotten fixed, but the legal issues have. Might not be related to the age of players being drafted, many explanations are possible. But it’s the league’s right to decide they shouldn’t be bringing 18 year olds into a sport often described as the world’s largest traveling brothel.
If they start paying players while they are still students, it’ll cause a horrendous amount of chaos. They are still too immature to handle being famous on campus, handling massive amounts of money will cause them to spiral completely out of control, disrupting the campus.