Is AT the corner acceptable too?
Yes. It should be acceptable.
idős - why isn't "elderly" accepted here?
It should be, please report it. Although idős has a wider range of application. :)
Does "sark" mean corner? I looked it up in two places. One didn't find anything, and the other one translated it to "polar." (??). "A sarkon" means "on the corner" - is that an idiom?
As far as i know, is sarok - corner.
Oh, thanks! I forgot that sometimes vowels get dropped when endings are added. Makes sense now.