It depends on the context.
'Jsou stará?' refers to neuter plural: 'Are (the cities) old?' = 'Jsou (ta města) stará?'
'Jsou staré?' refers to feminine plural or masculine inanimate plural: 'Are (the young ladies) old?' = 'Jsou (ty slečny) staré?' or 'Are (the forests) old?' = 'Jsou (ty lesy) staré?'
Or it can even be 'Jsou staří?' which refers to masculine animate plural: 'Are (the snowmen) old?' = 'Jsou (ti sněhuláci) staří?'
Hope it helps and happy learning! :)