Is there anything other than voting showing age of adulthood at 21?
The minimum legal drinking age (
MLDA) used to be 21 in all states to coincide with the minimum voting age.
When the voting age was lowered in 1971 to 18 years old, "29 states lowered their MLDA to either 18, 19 or 20 between 1970 and 1975. The reason some states acted before 1971 is that Nixon signed a law passed by Congress to lower the voting age, but it took a couple of years to get the SCOTUS to rule that, while a federal law can affect federal elections, it could not force states to lower the voting age for any state or local elections. The Constitution had to be amended. That change was ratified in 1971.
"In 1978, after reports began showing an increase in alcohol-related car accidents
involving drivers 18-20 years old, Michigan became the first state to reverse course
by raising its MLDA back up to 21; 17 states followed suit between 1980 and 1985.
The federal government enacted the National Minimum Legal Drinking Act in 1984;
this act required states to prohibit anyone under the age of 21 from buying or publicly
possessing alcohol. If a state failed to comply with these regulations after 1988, the
federal government would have the right to withhold 10 percent of the state's federal
highway funds. Unsurprisingly, all states had a MLDA of 21 by the end of 1988..."
https://www.ranker.com/list/us-age-restrictions/tracey-grahamNC raised our MLDA for beer & wine from 18 to 21 between 1981-1983 -- one year added annually to let those already legally able to buy alcohol continue to be legal. Hard liquor was always restricted to 21 year olds and had to be sold through ABC (Alcoholic Beverage Control Board) stores. Beer and wine was available in grocery and other retail stores.
I remember on a flight from NC to Texas in 1982, the stewardess (what's called a flight attendant now

) told us ROTC cadets we could buy any alcohol onboard while they were in the air as long as we were "on orders". The federal drinking age then was 18 for everything, so they followed federal law as long as we were under federal jurisdiction.
The federal MLDA was raised to 21 in 1984.
https://en.wikipedia.org/wiki/National_Minimum_Drinking_Age_Act