Either you are an adult at 18 or you aren’t. Pick one.
IF you can sign contracts at the age of 18, join the military at that age, get married without parental consent, vote at 18, etc. then that is the age that the law says you are a functioning adult.
Therefore, you should be able to buy beer, wine and sprits at that age, purchase (and carry) a handgun at that age, and in all ways enjoy the rights and privileges of being an adult. No other ages for certain things like buying booze and carrying a handgun. Either you are an adult at 18, or you aren’t. Choose one.
If not, then you should be kept as a legal child until age 21 or whatever age it is that these things become legal for young adults. I have no real objection if the law should be changed to 21, or some other (reasonable) age, as long as ALL the rights, responsibilities, and privileges convey at that same time. One age for adulthood.
I will now present the logical argument against your position.
Aaand, there isn’t one..
Make it 31.
The allow teenagers to join the military because they are young, gullible and believe they are invincible. If they waited a few years NOBODY would join because people eventually grow up and realize they are mortal.
They LEFT pushed to lower the voting age to 18 because they know that teens are young and gullible. They believe the socialist bullshit the left peddles to them and vote for it.
In both instances the selected age serves the agenda of those who made the decision.
You forgot one; a guy can be DRAFTED at 18!
I personally would not want to see the “age of accountability” raised to 21. That would mean another three years the PARENTS would be responsible for the stupid shit the “KID” did!