There will be lots of teens that age on the cruise. My DD's are now 16 and 19 and there have usually been 200+ teens on each of our cruises.
Be sure to register at Common Grounds on embarkation day. Your 18 year old may be interested in seeing if they will allow him to participate (Common Grounds is age 13-17) as they did accommodate my DD previously when she was that age.
The Teen Navigator will list the activities and locations which will allow them to pick and choose what they would like to attend. The 18 year old will be allowed in the adult areas of the ship, but drinking age is 21 on board.
My girls adore the cruise, and we are looking forward to cruise #4 in July!!