Why can't I "Jsou staré ženy"?
Why "Ony" and not "Ona"? I thought feminine plural should be treated as neuter.
It should not. Neuter is ona.
Why is "Jsou ony staré ženy" not accepted?
It is a bit strange. The word order is like a question.
If you ask why "Jsou to..." is not like a question then all I can say is that "To jsou.." "Jsou to..." is a very common sentence pattern which has its own rules.
Can you also say "Ony jsou staré ženy"? (I made another typo and got it wrong, but this is what i would have typed.
"Ony jsou staré ženy" is accepted.