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.

• "His cousin was now of more than middle age. . . . She was lean, and yellow, and long in the tooth."
• "So as Microsoft began its 30th year last month, investors wondered whether it's a little long in the tooth."
• "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."
• "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
• elderly

• don't look a gift horse in the mouth
• make old bones