I'm under 25, why shouldn't I use retinol?

Under 25, your skin is usually still producing ample collagen and doesn't need the powerful effects of retinol. When you are under 25 you have a high rate of skin turnover, meaning you already have efficient skin regeneration. 

Overuse can lead to irritation and isn't typically necessary for younger skin. There is also a risk of over-exfoliation, which can damage the skin barrier, leading to increased sensitivity, breakouts, and other skin issues. 

For younger skin focus on hydration and sun protection instead. These include ingredients such as hyaluronic acid and SPF.