The 7 year rule.

Over a 7 year period every cell in the human body is replaced. Unfortunately after a certain amount of years they begin to deteriorate but hey, technically speaking no one is older than 7 years old.

Next time you get asked “How old are you?!” by some condescending old moody bat. Pick a quiet, vulnerable pose and simply flip back “Which cell is it you are refering to? Maam?”