I don't nessessarily think that it'll regulate anything. Like alcohol, everyone has drank underage before. Even if it's 21 and over, theres someway of getting their hands on alcohol.
However, its the people who sell it. They get all shady about it, want to make an extra couple hundred bucks so they start selling it to ppl who can't get it otherwise. I remember I used to buy beer when I was 19, is it the law? or the person who sold it to me?