Chatty old people drive me nuts in a grocery line (probably because although I am getting older I am not at all chatty). They always get in front of me in line and have nothing better to do than tell the cashier about their second cousin's great niece's cat while my bananas are turning brown sitting on the conveyer belt. If you see me behind you sliding to the front of my cart and scooting my groceries forward on the belt by half an inch it is time to put away your pictures and go home.
Don't be an irritable inglubion! That may be the only interaction with a human that old person had all day long.
Be patient, you may be that old person, some day, and you'll appreciate the patience of the folks in the line behind you.