This is an old one too, if you have heard it before let a couple of people viewing it for the first time have a go before you post.
Apparently the following is carved on his tomb:
"God granted him to be a boy for the sixth part of his life, and adding a twelth part to this, He clothed his cheeks with down; He lit him the light of wedlock after a seventh part, and five years after his marriage He granted him a son. Alas! late-born wretched child; after attaining the measure of half his father's full life, chill Fate took him. After consoling his grief by this science of numbers for four years he ended his life."
What age was Diophantus when he died?