long in the tooth

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.

to be very old Don't you think she's a bit long in the tooth to be a romantic heroine?

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]

Growing old.

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.”