long in the tooth

Old. Animals' teeth, especially those of horses, are thought to be an indicator of age. As animals age, their gums recede, and their teeth look longer. Our poor cat is so long in the tooth that he struggles just walking around the house these days. She's a little long in the tooth to still be working—do you think she'll ever retire?

Fig. old. That actor is getting a little long in the tooth to play the romantic lead. I may be long in the tooth, but I'm not stupid.

Getting on in years, old, as in Aunt Aggie's a little long in the tooth to be helping us move. This expression alludes to a horse's gums receding with age and making the teeth appear longer. [Mid-1800s]

If you describe someone or something as long in the tooth, you mean that they are getting old, often too old for a particular activity or purpose. I'm a bit long in the tooth to start being a student. Their cars are looking rather long in the tooth, with the last model launched over 10 years ago. Note: This expression refers to the fact that you can judge the age of a horse by looking at its teeth. As horses get older, their teeth look longer because their gums are receding.

Growing old.

Aging or old. This unflattering term alludes to the fact that a horse’s gums recede as it gets older, and transfers the same phenomenon to humankind. The transfer understandably is not very old, since until relatively recent times adults who were old enough to experience gum recession generally had lost most or all of their teeth. It dates from the nineteenth century. Thackeray used it in Henry Esmond (1852): “She was lean and yellow and long in the tooth.”

Old. Absent conclusive documentation, a horse's age is determined by the size and condition of its teeth, which show specific signs of growth or deterioration over the years. For example, a groove in an upper incisor usually first appears when a horse is ten, moves halfway down the tooth in five years, reaches the end in another five, and then begins to disappear. There are far more flattering ways to refer to someone as being “long in the tooth”—to the extent that any reference to age is flattering—such as the French euphemism “a woman of a certain age.”