What is the origin of the idiom "long in the tooth" and when to use it?

The idiom "long in the tooth" has its origin in the practice of estimating a horse's age by examining its teeth, as horses' teeth grow longer as they age. Today, it is used figuratively to describe people who are older or show signs of aging. It can refer to a person's physical age or be used metaphorically to describe someone who seems outdated or out of touch, often employed in informal contexts to humorously highlight someone's age or experience.