I have stayed at both Nidd and Thoresby. I am in my early 50's with limited mobility and my husband is a similar age but very active. We really enjoyed both our stays. You can do as many or few of the activities as you like, there is no pressure but you are advised to pre-book the activities that can be pre-booked, although we had no problems at the time.
I personally prefer Thoresby overall, the building is just staggeringly gorgeous and there are so many places to enjoy it from. We stayed in the new building and it was too far for me to walk to the old building even with my walking aid, so after a couple of days I was exhausted, but I did have a lovely pedicure and swim. My husband on the other had went running in the woodland and joined in all sorts. Knowing this though, we are now booked in for a room in the old building and I can't wait.
Nidd was again fantastic, my husband particularly enjoyed the larger range of free activities there. We stayed in the new building, but booked by phone to discuss best locations and were given excellent advice so I managed fine this time. The only downside, in my opinion at Nidd, is the lack of choice of entertainment venue in the evening. It has the one combined bar and entertainment room, with no separation between them, and I did prefer having a window between the two at Thoresby (as well as a totally separate bar area). I am a bit greedy that way - I liked to see the show but a bit quieter too!
Food was the same at both and we enjoyed it, we even didn't mind that the starters and desserts were pretty much the same each night once we realised this was the case - my husband had the same dessert every night!
The age demographic is mainly older but everyone is there for a good time and let their hair down. Personally I couldn't keep up with them, everyone else seemed to have far much more energy than me! I suspect after noon naps may help, but we keep saying we are going to try to pace ourselves better - but we also don't want to miss out on anything!