Major component of skin that gives it flexibility. The process of making elastin in the body is known as elastogenesis. This begins during fetal development and continues in infancy and early childhood. While we're still young, elastogenesis drops off sharply and by the time we reach adulthood the body is no longer producing elastin in what would we considered a helpful amount (if it's even producing it at all). There are different types of elastin, all of which bundle to form a complex, mesh-like network of support that interweaves with skin's other supportive element, collagen as well as skin-repairing substances such as hyaluronic acid.
Sun damage causes elastin in skin to deteriorate and eventually break, which leads to sagging and skin losing its ability to bounce back when stretched. Under a microscope, sun-damaged elastin resembles a shapeless blob rather than a tightly linked network or support fibers (proof of just how bad sun exposure is for keeping skin's elastin intact). Because adults cannot generate new elastin to repair what's been damaged, skin care research is looking at ways to topically incite new elastin production, but so far the results haven't been all that impressive.
Key enzymes in skin sweep in to help protect elastin from damage, including from other enzymes in skin that are triggered by sun exposure and seek to destroy elastin fibers. However, once the helpful enzymes are depleted, elastin loses much of its ability to protect itself when faced with damaging UV light.
Simply getting older also causes elastin to degrade, often as much as 50%. This is one reason why people over age 80 have sagging skin even in areas that rarely see daylight. Think of elastin as a rubber band that can only be stretched so many times before it weakens and eventually snaps. You can repair the rubber band to some extent, but it will never be as resilient as it once was.
It's interesting to note that stretch marks on skin aren't scars per se. Rather, they are visibly broken bands of elastin. That's why stretch marks are so difficult to repair and why they look the way they do.
Elastin used in skin-care products is derived from both plant and animal sources. It functions as a good water-binding agent, but that's it. Elastin in skin-care products has never been shown to affect the elastin in skin or to have any other benefit, such as firming or lifting.