When we don't moisturize our skin, it loses collagen and starts melting down: it gets dehydrated, wrinkles begin to appear because there's no support, and cells don't reproduce themselves anymore. That's what we usually call "old skin." It's often also a very sick skin by a quick look because it's dry, and for this reason, it's not healthy: our body is made by up to 60% of water, and every single part of it needs water to stay alive. When we don't drink enough water, we feel tired, and the same happens to the skin: if we don't provide the skin with nutrition, it will die.