Why is it you eyelashes become shorter as you grow older?

An aging thing. Hair follicles all over the body go through about a 3 month cycle in which the hair grows for about a month, then stays for another month before falling out and the follicle "rests" for a month. As we age the growth phase gets shorter, thus the hairs do not grow as long before they stop growing. Prostaglandin eye drops for glaucoma and "latisse" appear to lengthen the growth phase.
Multiple reasons. Hair follicles do have a shorter growth phase as we get older, however other disease such as hypothyroidism and blepharitis can cause there to be damage to the hair itself. In addition there can be breakage from makeup and manipulation such as eyelash curling and chemical treatments like dyes.