Only those 21 will be able to buy it legally.
As it is, those under 21 are buying pot now, only at a higher rate because they’re buying it from drug dealers who will sell if to them along with other drugs the dealers try to get them hooked on.
If pot becomes legal, licensed stores will be where people who are 21 will be able to but it, and the drug dealers will be margenalized.
Sure, those under 21 will still buy pot, just as they buy alcohol, but it’s more difficult.
Jim