He simply wants to be able to legally drink. He’s saying he’s an adult legally in every way except drinking, and he thinks that’s unfair. The argument seems clear to me.

I think possible counterarguments might include a disproportionately heavy cost of his age group drinking, like higher DUI rates and more deaths on the road. It’s also possible that drinking could harm the physical and mental health of his age group more (just speculation; I have no evidence to support that, but someone else might).