In 1965, I graduated from high school, having lettered in cross country and track so I was in excellent shape. My mom told me to finish college before I did anything, so I enrolled. At that time there was a draft, but you could get a deferment for college. The draft board lady called me and was miffed that I chose college over the draft. “We’re gonna get you, » she told me. Meanwhile my friends who dropped out were sent off to Vietnam and killed, so I was in no hurry.
When I graduated in 1969, I had switched to rugby, so I was still in excellent shape. “We got you,” said the draft board lady, and off I went.
But I had studied languages in college, maxed the language aptitude test, and volunteered for Vietnamese language school. Plus as one of only 2 college grads in my basic training company, I was far more mature and good at everything than the typical 18 year old draftee from the Appalachian hills or the inner city. So not only did I ace all the tests, but won the company PT test.
Then I had a let us say interesting job in the Nam as a field interrogator with the First Cav on the Cambodian border where I could communicate with everyone including villagers and enemy prisoners.
Age 21 in the Nam. In those days you were going whether you went straight to college or not.
And the GI Bill worked out as well. When I returned I used it to go to law school and launch a career in law.
So I would recommend going to college first if you can afford it. You can be equally in shape at 21 as 18. And you can use the GI Bill for grad school as well as undergrad. And finally, you will be way more mature at 21 than 18.