Thin lips occur for the same reason skin begins to sag. As we get older, our bodies produce less collagen. Over time, lips become thinner, losing their youthful plumpness and volume. The size of your teeth plays a big role in how thin your lips may become as well. If you have small teeth or if your teeth are positioned in a certain way (such as in the case of an underbite), your lips may look thinner as a result.