If you were to select a cruise line by the average age of travelers, I would suggest the following:
10-45 Royal Caribbean, NCL
40-75 Holland America
Of course this is just my opinion of average ages. Someone else would have a totally different take on it. There are a lot of other reasons to select one cruise line over another but my best guess would have you on RCL. He will have a great time on Millennium and if he likes beaches and activiy, almost every port has plenty of opportunities with Tortolla and Martinique being the only dull stops for my taste and probably more so for young people.
My wife and I sailed Millennium on her eastern Caribbean intinerary in 2005. There were plenty of young people and even teens on the ship who seemed to group together around the pools.