At age 20, skin is replaced every three weeks. But by age 50, skin replacement slows down to every nine weeks. When skin cell replacement decreases, skin becomes thinner and more fragile.
Because of scars, sun damage, oxidative damage, damaged proteins accumulate in the skin.
Around age 25, skin oils starts to decline. This causes a dryer skin. After age 45, it becomes more serious.
After age 25, the skin slows down to generate collagen, elastin, and the water-holding GAGs. Therefore, the wrinkle appears, and elasticity begins to lose.