This was my first cruise and overall it was a great experience. I'm in my early 30's so definitely younger than the overall crowd but it was a family trip with my parents who are frequent cruisers. Here are my overall thoughts:
Ship: As other reviewers have mentioned Radiance of the Seas has an older ship feel. Everything is clean and well maintained but the overall decor / style is outdated and looks like it's from the 80's. It's also lacking a lot of the modern amenities / features of the newer bigger ships. I was surprised how small the shopping area and pool area were but there is still enough to keep you entertained for the week. The solarium area had such a strong chlorine smell that I couldn't spend anytime there but it didn't seem to bother other people. It never felt overcrowded which is nice for a smaller ship.
Activities: I never found a lack of things to do. There is something for people of all ages but overall the activites are geared towards the 50+ market. If you like to party this might not be the cruise for you. I enjoyed playing mini golf on the top deck and arcade games. I used the gym most days and there's plenty of equipment. There is a cinema showing movies three times a day and we also watched a movie from the outside hot tub. We had fun at bingo and trivia. We are not big drinkers but there are plenty of bars and a good cocktail selection.