The phrase is usually along the lines of a little bit older than my teeth as per QM's answer, because generally, we are not born with teeth but get them (our adult set) a few years after.
I don't think it has any connection with determining age from teeth as per horses.
The version DrXXX heard seems to have suffered the mangled metaphor syndrome along the same lines as "cheap at half the price"
Once intended to be a deliberate and sarcastic corruption of the original 'cheap at twice the price', it now increasingly and incorrectly seems to be used in place of the original phrase.