Great question. At some point in life, generally by age 30 or so, the fibroblasts in your skin that make collagen go dormant. They are still there in your dermis, but it takes stimulation to make them wake up and make collagen. Meanwhile, your body breaks down collagen at 1-2% per year or so. So throughout your adult life, the dermis is slowly thinning out. This is why we tend to start noticing skin changes sometime in the late 30's or early 40's. Collagen and elastin are what keep your skin thick and springy, and resistant to gravity. Once it thins enough, you see wrinkles and sagginess. But even at age 70, you can wake those cells up and make collagen, thickening and tightening your skin. The most powerful device for this right now is Infini, however, any device (laser or RF) that gets down to the dermis can stimulate collagen.