If they are set on Negril then Riu Tropical Bay is a good choice, but if they will consider Runaway Bay ... my kids loved Jewel Runaway Bay when they were younger teens (it was BRB then), when they were 18 and 16, and still loved it last time at ages 21 AND 19. They would go back in a minute. We did Riu Tropical Bay when they were around 14 and 12 and we liked the resort but there were far fewer activities and they were bored a bit at times. As for Beaches, we took then to Beaches Negril once and Beaches Ochos Rios (Boscobel) twice and they had a great time. Beaches does cater to younger kids, but there is plenty to do for older teens and young adults too!
All the best -
Roy