You need to ba 18 but you can join the ROTC or a military group for younger people but it is not technically the "Military"
Yes
Within the US Military that tradition is deeply ingrained and accepted.
He was invited in to the group and he accepted the offer
They were accepted
If you are accepted to BYU they will mail you an acceptance letter just like any other university.
Yes, but the children mostly come from military families.
The accepted abbreviation is 'ops', both in military and hospital usage.
The academy I hope to attend is Pratt Institute. I hope to be accepted at a military academy.
Militia Act
The military is not going to let you join up if you have had a major organ transplant. If you were in the military at the time, it might be accepted, I know that kidney transplants are allowed to continue service.
17 with a waiver
No. He was probably only 17 at the time of Marathon in 490 BCE, and young men were not accepted for military service until 18. He is recorded as fighting at Salamis during the second Persian invasion in 480 BCE.