Why stara? adjectival plural ending is normally e. [with accent]
Because zvířata are neuter.
Masculine variant nominative "starý" ends with "-rý" and therefore it follows a hard pattern "mladý".
Why not "Ony jsou stará zvířata?" or "Jsou ony stará zvířata?"?
Doesn't zvíře become feminine when it goes to plural?
No, it does not. I know only one word that goes from neuter to feminine in plural - dítě.
Oko, ucho - if we mean body organs.
That is correct I found out since and commented so elsewhere. I do not think such an omission warrants a downvote though...
What is wrong with "ony" being included in the sentence?
Ony is feminine, zvířata is neuter - ona.
isn't "ta" used for "those"?
For "those animals" yes, that would be "ta zvířata".
But this is the so common "to je/to jsou/je to/jsou to" pattern "that is/those are". We have the same question almost every day.
"We have the same question almost every day."
HA, protože to je matoucí
Isn't "to" for singular neutro?
In the "je to"/"to je"/"jsou to""to jsou" you always have "to" for both numbers, all genders and even all persons (To jsem já, To jste vy, To je zvíře, To jsou zvířata).
Could you also have put "To jsou zvířata stará?"
No, an agreeing modifier shall come in front of the noun it modifies.
What is the difference between to and ta
Check my answer to goldregenb and DiegoPlat7.
Ta is feminine singular or neuter plural.
Why did they accept oni jsou stari zviřata as a good answer?
There is no accepted answer with "staří zvířata". Only "staří živočichové". We would need a screenshot from you.