* As horses age, their teeth become longer and more worn down. This is because they use their teeth to graze on tough grasses and other vegetation, and the constant wear and tear causes the teeth to grow longer.
* The length and appearance of a horse's teeth are a good indicator of its age. Older horses have longer, more worn-down teeth than younger horses.
* The phrase itself likely originated in the 17th or 18th century. The specific origin is unknown, but it was likely used by people familiar with horses and their aging process.
Therefore, "long in the tooth" came to be used metaphorically to describe someone who is old or elderly. The idiom highlights the connection between age and the physical changes that occur with time, such as the wearing down of teeth.