long in the tooth

Possibly from the practice of examining the length of horses’ teeth when estimating their ages: an old horse has long, rectangular incisors, and their occlusion angle is steep. Compare don't look a gift horse in the mouth.

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long in the tooth

1. (idiomatic) Old; aged.
   • 1852, William Makepeace Thackeray, chapter 2, in The History of Henry Esmond, Esq.:

     His cousin was now of more than middle age. . . . She was lean, and yellow, and long in the tooth.

   • 2004 May 10, Chris Taylor, “Is Microsoft A Slowpoke?”, in Time‎, archived from the original on 6 April 2008:

     So as Microsoft began its 30th year last month, investors wondered whether it's a little long in the tooth.

   • 2019 March 13, Drachinifel, 10:25 from the start, in The Russian 2nd Pacific Squadron - Voyage of the Damned‎, archived from the original on 16 December 2022:

     There were four relatively-fast, modern cruisers, the Oleg, Aurora, Zhemchug, and Izumrud... aaand the Dmitrii Donskoi, which was twenty-one years old and getting a bit long in the tooth.

• on in years, over the hill, wizened; see also Thesaurus:elderly

• don't look a gift horse in the mouth

• make old bones