All things being equal, if the Govt decides a teenager becomes a legal adult at 18 then ALL adult privieges should be given to them.
Arbitrarily selecting 21 as the legal drinking (and some States, smoking) indicates that there are two levels of adulthood.
Now, should the Govt decide that 21 is a legal adult then NO ONE should join the services until they are 21.
That's my opinion on that matter.
As it is now, the Govt wants it, and gets it, both ways.
Me. FWIW, I joined the Army at 17 back in 1972 and served 37 years.