Im thinking of booking in the middle of May. We are spending 2 weeks in Maine and we have never been to that area. We are 65 and in average shape. I would think that would be off season is everything closed. How cold is it?
Mid-May on the coast of Maine where I'm at is kind of a crapshoot. It can be beautiful, or it can be cold and miserable. No snow, of course...you'll be past that! I am an eternal optimist, as tonyg and some of the others will tell you...so I'd say just do a little more planning. Many of the museums stay open year round, the ocean is beautiful no matter the weather, and the absence of leaves on the trees doesn't affect the mountain vistas.
We've spent many April School Breaks at Smuggs, and are considering that next year as well. Great even in the off-season! Pack layers, and travel!
Where are you going in Maine? That might help us offer more specific advice. Hope you'll enjoy my home state!