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^[1], 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^[2], 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.

   • 2024, Jeremy B. Rudd, A Practical Guide to Macroeconomics, p. 47

     For those who are interested, Deaton (1992) remains the best (and most readable) single introduction to the empirics of the canonical permanent income model, though it's now 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