I sailed the Regal to Alaska last May with my 3 nieces (ages 3, 8 and 9) and my nephew (age 16). The three girls loved the kids program and seemed to notice no difference between the larger ships we have been on (Sapphire and Dawn).
My nephew, however, had a more difficult time. There seemed to be way less structured activities for the teenagers and they more or less just roamed the ship in packs. He is a little shy and has trouble making friends outside of a structured environment. On the larger ships, there were lots of activities to help him break the ice and feel comfortable, but on the Regal he was kind of left on his own and I don't think he ever really got to where he felt comfortable with the other teenagers. If your 15 year old is more outgoing, then this probably won't be an issue for you as the other teenagers seemed to be having fun just hanging out.
One problem we did find with the Regal concerning the kids program was the location. The kid's center is located at the top of the ship and the only way to get to it is by walking down an outside corridor and up an outside staircase in what was often very windy conditions. It was sometimes quite scary navigating the little ones to and from the center. They actually had to close it the entire last day of the cruise (talk about 3 very disappointed kids!) because the winds were too bad for it to be safe going up the staircase. Surprisingly, they just canceled all activities rather than moving them to a better location.
Personally, I prefer the bigger ships, but the Regal is still nice and I would sail on her again if the price and itinerary were right.