We did the Thanksgiving western last year with my parents who are in their early 70's, and didn't really have a problem. Our DS12 went with them on a couple of excursions (one in Cozumel and one at CC) and they all had a great time. While they were off on their excursion in Cozumel, DW and I did some shopping and then just relaxed on the ship. At CC, we met up with them at the entrance to Serenity Bay (where their excursion ended) and all went back to the family beach for lunch and playing in the water. All in all, it was a good trip and I would consider doing it again.
As long as your parents understand, up front, that you are not going to be "joined at the hip" and doing everything together, you'll be fine.