Illegal drug dealers have very little incentive not to sell to underage kids, while legal ones have a license and livelihood that could be taken away.
Of course legal drugs always have “cool” adults willing to be straw purchasers for underage kids. It’s complicated, and the relationship between the law and underage usage are non linear.
It'll probably remain at least slightly easier to get drugs than alcohol even if/when both are legal to 21+ just because drugs are so much smaller and easier to deal with. A trunk full of Alcohol isn't much but a trunk of any drug is a lot.
Of course legal drugs always have “cool” adults willing to be straw purchasers for underage kids. It’s complicated, and the relationship between the law and underage usage are non linear.