Older animals have worn, smoother and yellower teeth. Whiter, smaller, sharper teeth are the sign of a younger animal. General muscle tone, flexibility, tenderness. A younger animal will retain more pliability even when dead, and a finger poked deeply into a haunch will cause the meat to dent in and then rebound more quickly. Ossification, thickening or fusing in the bone joints, thick, toughened pads on the bottoms of the paws and the leg joints or a protruberent spine and shoulders can signify an older animal in poorer condition.
Age matters somewhat less than diet and condition.
But even an old, tough squirrel has a lovely sweet flavor that can be beautifully brought out in a simple stew with lemon juice, spring lily bulbs or fresh garlic greens and potatoes. I actually like old, flavorful squirrel stew as much or more than tender young roasted or grilled squirrel. If in doubt, cut off a small piece of meat from the hindquarters and fry it very briefly in a small amount of oil. Bite into it when it's piping hot and seasoned only with a dash of salt and pepper. Tougher squirrels benefit from old fashioned stew and simmer recipes, while tender young'uns can be used very much like fryer rabbit in a number of creative dishes.