Nail polish becomes thicker with age due to evaporating solvents, according to Daily Makeover. When these solvents evaporate, the polish gets thick and stringy, resulting in lumpy, uneven results. Thick polish also does not adhere to the nail bed very well, which causes chipping and peeling. For best results, Daily Makeover says never to use polish that is more than 12 months old.
There are many things that can speed along the evaporation process and cause nail polish to become too thick before its time. Leaving the lid off the polish for long periods of time can dry out the polish. For this reason, polish should be closed tightly whenever the brush is not in use. Allowing dried polish to accumulate along the neck of the bottle can allow air to seep into the bottle even when it's closed. It's always a good idea to clean the neck of the bottle after each use.
If polish becomes too thick to work with, a few drops of polish thinner may thin it enough to make it usable once again. Available wherever nail polish is sold, nail polish thinner may lengthen the life of a bottle of polish up to three months or longer, depending on the brand and quality.