Long in the tooth means 'elderly; old', and by extension 'past one's prime; over the hill'.
The expression derives from horses, of all things. As horses age, their gums recess, which is the origin of the practice of examining a horse's teeth to determine its age (and therefore value), a sort of early version of kicking the tires. (Horse traders, like used-car salesman, are not to be trusted.) An old horse's gums will have recessed so much that the roots of the teeth are visible, thus making the teeth appear longer--hence long in the tooth to mean 'old', and hence used figuratively paralleling figurative senses of 'old' such as 'past one's prime'.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.