Is no language with the non-primes property context-free?
You can use Parikh's theorem to prove this claim.

Indeed, if such a language was context-free, then its Parikh image would be regular. But the Parikh image would also satisfy the condition (as it only talks about length of words). It's very easy to prove that no regular language can satisfy this condition (e.g. by complementing it and using the pumping lemma).