Hi!
My son also got a
Disney Cruise for his birthday/graduation present. He turned 18 when we sailed. As PP stated, 18 year olds who are still in high school or haven't started college are allowed to go to The Stack BUT....
The counselors at the stack told my son that he had to choose whether he'd participate in the stack activities during the week or he would hang out with the College Club kids (18-22), he couldn't do both. After visiting the stack, he decided to hang out with a group of friends he met at the College Club meet. They did a TON of stuff together, played basketball, went to the movies, went to the adult pool. One day, Dave, one of the CM's told them that he was organizing a pool game and they all BETTER be there, it was a lot of fun for them.
At night they went to the lounges and watched the shows and then they all hung out on deck 9.
Allowing 18 year olds in the stack is up to the counselors (this is what
DCL told me), but the chances are good, specially if you want to hang out with your brother.
That said, he won't be able to join you in the adult pool or in the lounges after 9 p.m. but if you like the activities in the stack, you are all set. There are also a ton of other activities you guys can participate in. It all depends what you like.
Have fun!