Answer:
I am longer than a decade and shorter than a millennium
I only exist when you are here. Where you never were I can never be.
I'm at the head of an elephant and at the tail of a squirrel
I am not a toy though many use me for pleasure. I have a butt but I cannot poo.
What always ends everything?
Strip the skin under my skin, and my flesh you'll reveal. It tastes sweet and tart, now throw out the peel. What is it?