If you ever watched The Golden Girls, and are twisted enough to wonder what if one of them was a Slayer - I am so not joking, it's AWESOME, which is why it gets its very own entry. I don't know how I haven't come across this one before now myself...

The Oldest F#*%ing Slayer, Ever by vinniebatman
PG-13 (complete) gen
Crossover The Golden Girls/Buffy the Vampire Slayer
Summary: At 58, she is the oldest slayer in history. But at least she's not short.
This story is No. 3 in the series "Slayers that Never Were... Thank God". You may wish to read the series introduction and the preceeding stories first.
(Rec Note: No, I haven't read the other stories in this series, yet. I will, but reading the preceeding stories isn't necessary to understand this one. Though you should at least have a vague idea of what happened in the last episode of Buffy.)