Graveyard is the answer.

Explanation:

Where in equestria are old bays left when used up?

A bay is a reddish-brown animal especially a horse having a black mane and tail. Used up another way of saying dead, that also works for black and red colors being very common colors for OCs. Old meaning either same old same old or old as in age and when death is but a inevitability.

Where do those who are one with the stars settle down for tea?

One with the stars being an analogy of being dead. Most religions have ascension to the skies as a theme of the after life, and stars are oft associated with night which is associated with dark and morbid things. Settle down for tea is just a colorful analogy for where a being resides.

2077707

2077707 I see. It seems a bit obvious now.

Good work. You managed to stump everyone.

P.S. I can see, however, where everyone was confused, as some of the analogies were very obscure. Good work, though.