While we stereotype grandparents to be old they don't have to be. post #7629094 for instance is a grandmother_and_granddaughter where neither would apply for old_woman. post #3230763 is another solo one in the tag I wouldn't say is visually old either.
perhaps grandma is supposed to be used for characters that are related in canon appearing together (like sisters), but with the lack of a wiki some unrelated posts got mixed in? in that case i think the tag could stand on its own given some cleanup. (though there's also grandmother_and_grandson/daughter and the accompanying grandfather tags)
@BingusX Don't blank out the content of a BUR if it's not going the way you want it to go, that's not okay. See topic #29393 (admin-reverted) and topic #27916 (user-reverted).