“Long in the tooth” comes from the centuries-old practice of checking a horse’s mouth to estimate its age. As horses get older, their teeth appear visibly longer, so a horse that was “long in the tooth” was an old horse. The phrase eventually jumped from the stable to everyday English, where it became a slightly unflattering way to call someone old.
Why Horse Teeth Actually Get Longer
Horses have a type of tooth called a hypsodont, or high-crowned tooth, that works fundamentally differently from a human tooth. A horse’s diet of coarse grasses mixed with grit and soil grinds down the chewing surface at a rate of about 3 to 4 millimeters per year. To compensate, the tooth continuously pushes upward out of the jawbone throughout the horse’s life, exposing fresh material as the top wears away.
This is possible because horse teeth are remarkably long structures, most of which sit below the gum line in a young animal. A young horse might have several inches of tooth still hidden in the jaw, waiting to erupt. As the horse ages and more of that reserve pushes through, the visible portion of the tooth grows longer and changes shape. The teeth also shift from oval to more triangular in cross-section and begin to angle forward. An old rhyme used by horsemen captures the progression: “As time goes on the horsemen know / The oval teeth three-sided grow; / Then longer get, project before / Till twenty, when they know no more.”
Gum recession plays a role too. Just as in humans, aging horses lose gum tissue, which exposes more of the tooth root and adds to the appearance of length. The combination of continuous eruption and receding gums made a horse’s mouth one of the most visible markers of age available to people who worked with these animals daily.
How People Used Teeth to Judge a Horse’s Age
For centuries, a horse’s value depended heavily on its age. Buyers routinely opened a horse’s mouth and examined the teeth before agreeing on a price. Several specific features guided the assessment: the depth of the “cups” (small indentations on the chewing surface that gradually wear away), the appearance of a dark spot called the dental star, the angle of the front teeth, and a groove on the upper corner incisor known as Galvayne’s groove that appears around age 10 and extends down the tooth over the following decade.
A study of 80 horses with confirmed ages found that dental aging is reasonably accurate up to about five years old, when the pattern of baby teeth being replaced by adult teeth provides clear milestones. After age 11, though, accuracy drops significantly. Individual variation in diet, genetics, and environment means two horses of the same age can have noticeably different dental profiles. The disappearance of the cups turned out to be one of the more reliable indicators, while some features, like a hook on the upper back incisor, showed up in horses of practically any age over six, making them less useful than tradition suggested.
Despite these limitations, tooth checks were the best tool available before modern record-keeping. The practice was so universal that it gave English not just “long in the tooth” but also “don’t look a gift horse in the mouth,” meaning you shouldn’t inspect the age (and therefore value) of a horse someone gave you for free.
When the Phrase Entered Common English
The earliest recorded uses of “long in the tooth” referring to horses date to the 1800s, though the practice of aging horses by their teeth is far older. By the mid-nineteenth century, writers were already applying the phrase to people rather than horses. It carried, and still carries, a mildly rude edge. Calling someone long in the tooth doesn’t just mean they’re old; it implies they’re past their prime or perhaps too old for whatever they’re attempting.
The idiom fits into a broader family of English expressions borrowed from horse culture. “Straight from the horse’s mouth” (reliable information, because checking the teeth yourself was more trustworthy than taking a seller’s word), “don’t look a gift horse in the mouth,” and “long in the tooth” all trace back to the same era when horses were central to daily life and knowing their age was a practical economic skill.
Does the Same Thing Happen to Human Teeth?
Human teeth do appear longer with age, but for a different reason. Human teeth are brachydont, or short-crowned. They erupt fully and then stop. There’s no reserve of tooth sitting in the jawbone waiting to push through. Instead, the appearance of longer teeth in older people comes almost entirely from gum recession. As gum tissue pulls back from years of wear, brushing habits, or periodontal disease, more of the tooth root becomes exposed.
The biology is strikingly different at the cellular level. The cells that build tooth material in humans settle into a resting state after the tooth forms, producing only tiny amounts of new material for the rest of your life. Horse teeth, by contrast, constantly replace those same cells to keep up with the relentless grinding. A horse’s tooth is essentially in a permanent state of self-repair. Human teeth don’t have that ability, which is why cavities and wear are permanent problems for us in a way they aren’t for horses.