why is it not ok to say : Eski yatağı nerede?
That would mean "Where is her/his/its old bed?" although I bet that you meant to use the accusative. The accusative is only for direct objects. Subjects of a sentence will simple never be in the accusative case. :)
Why can't we say yaşlı yatak nerede?
Yaşlı is used for people, eski for things.
Because yaşlı is used for people and eski is used for things (I think)