Wordorigins.org

To be long in the tooth is to be old. It is a reference to gums receding with age and was originally applied to horses, as a way of gauging the age of a horse. The phrase dates to the mid-19th century when it appears in Thackeray’s The History of Henry Esmond in 1852:

She was lean, and yellow, and long in the tooth; all the red and white in all the toyshops of London could not make a beauty of her.

(Source: Oxford English Dictionary, 2nd Edition)