I suppose the minimum age to join the military should be 25 as well.
Change the voting age too.
You should have at least some life experience before voting so you know what you are actually voting for. When you are young, free government stuff sounds good. As you get older, you learn free isn’t free and you are the one actually paying for it via taxes.
Voting, age 25.
Military service 25.
Homeownership 25.
Age of sexual consent/marriage 25.
Consumption of alcohol 25.
Gun ownership, etc.
However, I don't agree with the ages, but I do agree everything should be consistant. One thing that really chapped my butt is my soldiers, 17-20 couldn't drink, but had been on 2-3 tours in the 'stan and Iraq, nor could they purchase a handgun for defense when home on leave. I think the best answer is like Heinlein said: Compulsary military service, 3 years, THEN you get citizen rights contained in Consitution.
Can't hack it in the military? Good. Go build a something in the Middle East or Central Asia for 3 years surrounded by people trying to kill you for your beliefs. If you make it back, you are a citizen with all rights granted! (*and these little narcissistic kids today that only care about Idol, Xbox, selfies, twerking, Mylie Cyrus, and JustaBeiber will be alot more appreciative of the US).