If you describe someone or something as long in the tooth, you mean that they are getting old, often too old for a particular activity or purpose.

long in the tooth Getting on in years, old, as in Aunt Aggie's a little long in the tooth to be helping us move. This expression alludes to a horse's gums receding with age and making the teeth appear longer.

To be long in the tooth is to be old, either in age or simply out of date. This phrase originated with horses, whose teeth continue to grow and be worn down throughout their life, so that by looking at their teeth one can guess at the horses’ age. It is commonly used in the financial and technological worlds where items can be dated very quickly.