Q. How does one prove that your not in your 20's until your 21? Now I would have thought that numbers run from 1-10 would be enough proof. But, what I'm up against are people that say that once you hit your 20th b'day then the next day that person is 1 day into their 21st year of life and therefore are in your 20's on your 20th birthday.
you are basically asking a semantics question rather than a maths one. Start by defining "in your twenties" then start counting
if i chose how everything worked, i would make "in your twenties" mean "in your 20th or 21st or ..... or 29th year of life", which would make you between 19 and 28... but i dont think my view is very popular