Hi there.
I too am finding that there are younger and younger people coming on the holidays which is great. My experience is that people of all ages on the holidays mix together well, and find plenty to talk about.
I have been on this particular holiday twice. If you have never been to Croatia before you are certainly in for a treat. The scenery is wonderful, the people are really friendly, and prices are till relatively low.
I will say that there is quite a bit of walking on this holiday, certainly if you go to the Plitvice Lakes which is now an included excursion. There is plenty of really nice walking in the hotel area too. The beaches are all pebbles so do take beach shoes.
I am more than happy to answer any questions about the hotel and the excursions. 🙂