English Idiom: Long in the tooth
We’re all getting a little long in the tooth. This idiom means that someone is old or at least older than you are. As far as I can ascertain, the expression comes from the fact that as some animals, such as horses, age their gums recede, and this makes their teeth look longer.
So the older a horse is, the longer its teeth will appear when compared to younger horses.
I suppose back in the day, many people had horses, and they then started to use the expression to refer to people.
It is not a rude expression, but I am not sure anybody would like to be told they are old or long in the tooth, so use this idiom wisely.
I tend to use it when I talk about myself and my experience. I have 20 years of teaching experience, and because of that, I have been in various teaching situations – one of the benefits of being long in the tooth, I guess.