I'm warming to this idea as well.
Anyone under 21 would be considered a minor. As it stands, can't even legally buy a beer until 21.
On the voting part, imagine all the support & votes the (D)'s would loose!
The only major issue with this scenario is that someone 20 years old (for example), couldn't get married then w/o a court order/parent consent.
However, perhaps a few things such as marriage and being able to rent/buy an apartment/house could be exempted.
But yeah, if guns go to 21, then at a minimum voting and military service age must be raised to 21 as well. If there is a declaration of war by congress, then the service age could be temporarily lowered to an age and duration as defined by congress at the time.
Ok, let’s raise the age of statutory rape to 18 as well. That should shake things up.