Welcome aboard, Donald! Disney Cruise Line is a cruise line for kids of all ages! Even I feel like a kid when I set sail with Captain Mickey, as Disney is amazing at not only planning magical children's activities but activities that are just as magical (if not more so!) for adults as well!
There are Youth Club spaces for kids from babies to 17-year olds. However, young adults like your children are not left out at all! They'll want to check out Club 18*21. While there isn't a dedicated "space" for this group, you'll find Club 18*21 events and activities listed in your Personal Navigator schedule each day that will take that group all around the ship. During college breaks, you'll see that group well attended, however, I've not seen it quite as well attended during months when most young adults are back at school. The best thing for your sons to do is look on that first Personal Navigator to find out when and where the first Club 18*21 social will be. The Cruise Staff Team that runs that group will be on hand to meet those young adults and plan great activities for the rest of the cruise with them. They won't want to miss out that first night.
Now, besides Club 18*21, since your sons are over 18, they are able to enjoy the nightclub spaces as well as other
adults-only spaces onboard the Magic. I have a feeling they'll love After Hours, the adult clubs, in the evening. Whether it's a low key night at Keys, the piano bar, or catching the latest variety act in Fathoms, where even later in the evening they can dance the night away, or board games and catching their favorite sports match in the sports bar, O'Gills, there is a space for them in After Hours. Plus, I bet they'll enjoy escaping from the little kids by relaxing and enjoying Quiet Cove Pool and Cove Cafe, a pool and bar area just for adults. Have fun as you continue to plan this vacation, Donald!