Your conversation made me curious so I looked up the history behind the drinking age:
Per this website, the national drinking age was set at 21 due to the danger of individuals getting killed by a drunk driver.
https://www.boston.com/culture/health/2014/07/17/why-21-a-look-at-our-nations-drinking-age
"During the late 1960s and 1970s, nearly all states lowered the drinking age to 18. This led to a huge increase in alcohol-related car accidents and drunk driving was deemed a public health crisis. In the mid-1970s, 60 percent of all traffic fatalities were alcohol related, according to the National Institute of Health(NIH). Over two-thirds of car accidents involving persons aged 16 to 20 were alcohol-related."
Essentially, it sounds as though the law wasn't put into place only to protect young people but to protect
anyone on the road from young drunk drivers.
I haven't noticed an epidemic of 16-year-olds stuck in unwanted marriages. I'd assume that the average age for most people getting married has gone up rather than down.
It should probably be noted that it
is more difficult for teenagers to get a driver's license these days than it used to be. So the hazards of teen driving are being dealt with in both ways.