"If the drinking age were 18, the only difference is that the 18-20 year olds would also have the option of binge drinking at bars."

You forgot to add: "where, of course, they'd at least be monitored by establishments with a vested interest in their safety and that of those around them, given the states' increasingly aggressive use of dram shop liability laws and the financially ruinous (or even criminal) penalties which attend them."

There, it's fixed.