Have you ever wondered how to fix thin lips? Lip size and shape are genetically determined, but all of us tend to get thinner lips over time. If your lips are naturally smaller, you may start to notice thinning of the lips in your 30s. This is due to collagen and hyaluronic acid production starting to slow down. As we get older, the rate at which our body produces this vital protein begins to slow. Once men and women reach their 20s, collagen is lost at a rate of about 1%-2% per year.