long in the tooth: to be very old.
Etymology: based on the idea that teeth grow longer in some animals as they get older
成语“马齿徒增”说马的牙齿随年龄的增长而增长,所以看马齿就可以知道马的年龄,后就用来比喻人的年龄.年龄白白地增加.常对人自称年华虚度而无长进,表示谦虚.