You can also "age" the meat in your refrigerator, putting it in bowls, and rotating it, and cleaning out the blood 2x a day for a week. That tenderizes, and removes some of the bloody flavor so many people find objectionable. Bone the meat, of course. The bones and fat, and connecting tissue contain enzymes that toughen the meat, and sour it. The young squirrels can usually be eaten the same day. Leave the big ones for the next weekend!
I once had some Squirrel Stew that was just wonderful, and all the squirrel meat was tender, and tasty. It had been cooked down in a pot for several hours, and wet recipes like that are the way to tenderize old squirrels, too.