I’m going to assume you haven’t gone down the lip filler route and your lips are ageing naturally, which means of course, that they’ve lost their natural collagen and are noticeably thinner.
Men, shakes fist at sky can simply grow a beard, et voilà, the whole lower half of their face is disguised.
Even young women with thin lips, especially a thin top lip, can’t get away with dark lipstick. So lighten up, you don’t want to draw any more attention to their lack of volume.
Find a shade that’s not too dark, maybe a deep fuchsia/rose-pink or a watermelon red (flattering and teeth whitening) and then buy another lighter shade in the same colour family – or even better, a not overly shiny lip gloss. L’Oréal Glam Shine Lip Glosses are ideal, especially in the shades Watermelon, Lychee & Grapefruit. Put the lighter shade in the middle of your lower lip and a little dab on your upper lip.
The idea of course is that the lighter shade will create the illusion of bringing that part of the lip forward, thus it will look fuller. It sucks to resort to silly tricks but it works.