If your eye shadow was a human, would it be old enough to enroll in kindergarten? Would your mascara be a toddler? It's hard to keep track of the age of make-up but not good to keep using it beyond when you should. Here is a great article from Good Housekeeping outlining how long you should keep various cosmetic products and why. They also recommend these fabulous stickers from Timestrips that help monitor the elapsed time that your cosmetic has been opened.