The short answer is loss of collagen and elastin caused by aging. The reasons are many but the top two are sun damage and hormonal decline with advanced aging, which is usually noticed after you approach and pass 50. But wait, it’s not only your skin that is changing but your muscles are also undergoing “sarcopenia”, which is a fancy way to say when your muscles lose their tone and elasticity. Other causes are recurrent weight fluctuation ,Yo-yo syndrome, metabolic changes like diabetes or thyroid issues, pregnancy, dehydration, and finally, the one we can’t help at all: genetics.