18 was considered too young to have alcohol but it’s still old enough to join the military, legally sign contracts, etc.

That's the reasoning that was used to lower the drinking age after the draft/Vietnam War

Also the reasoning used to lower the voting age at the same time, with the 26th Amendment.

but not old enough to rent a car at market rates (25 in U.S.)

Hertz and Avis were well ahead of the game.

https://www.npr.org/templates/story/story.php?storyId=141164...

When I was on my first business trip at 18, I couldn't check into a hotel in Michigan by myself. I had to call me contact at (then) DaimlerChrysler to meet me for check in. I had a credit card that could buy a brand new car, but I couldn't get the key to a $75 hotel room due to age.

I later found out that, at the time, there was no law preventing me from acquiring the room, but the hotel policy was to not allow unaccompanied individuals under 21.

And insurance companies had enough sense to charge an arm and a leg to a single 24 year old with a Mustang. My insurance cost more than my car note back then.