Does there ever come an age for a rabbit where it would be best if they stayed indoors or are all ages suited to winters outside?

I think if any of mine got to the stage where they became part immobile, I'd bring them inside regardless of their age.

It depends on the pysical condition of the bunny. A very old frail or diabled bunny may do better indoors, but if they have been used to living out, they might not enjoy being inside. I would treat every case as individual.

I think it depends on the age and general health of the bun. I think they must be like us when we get old - we tend to feel it more. But as long as the bun is happy enough.