Lips begin to thin as you get older due to, like your skin, a natural decline in collagen and hyaluronic acid, which keeps your lips moist and plump from the inside out. Also, like most other things, genetics play a part in determining the plumpness of your lips but everyone’s do get thinner over time. Also, lifestyle factors can be to blame for lip thinning such as too much sun exposure, dehydration, smoking and even drinking out of a straw. Anything that repeatedly causes creases in your lips will break down the collagen resulting in deeper lines and less fullness.