Is it a good idea to arrive a day early before a tour of Croatias coast. I'm 70 and in good shape but think the body needs to adjust. If so, is it easy to get a room for one night? Is it difficult and costly to get to old town from the airport?
Absolutely arrive at least a day early.
What airport are you flying into, and where does your tour start?
Well, I certainly think it's a good idea. My wife and I are doing an organized hiking trip (we're a shade younger than you, but not much) and we're arriving three nights before it begins--which is a combination of trying to get over jet, but also because with flight delays our scheduled flight may not get to Durbovnik when we think it will. To me, the very fact that you're asking this question means that it's a potential worry for you and that you should get there in advance.
Since you're on a tour, can you call the tour company and see if they can get you in the hotel that you start off with a day early? If not, it shouldn't be that difficult unless you're going peak season...