Definition: Elderly, past your prime.
Example: At the age of ninety-six, Herbert soon discovered he was a bit long in the tooth to dance the Macarena.
Long in the tooth is one of the few idioms that Iddy has found a definitive origin for, at least so far. It comes from the horse trade. The older a horse gets, the more its gums recede, making its teeth look longer. Thus, a horse that is ‘long in the tooth’ is old.