I don't really agree with Naans. I'd rather say that this "de" is everything but precise... As I said earlier, for your other question, you can use "aucun" instead of "de" in those negative sentences. "c'est vrai que nous n'avons mis aucune limite d'âge" So I would say that this "de" is here to say that they're not talking about some precise "limite d'âge", on the contrary, they're saying that they didn't "mettre" ANY "limite d'âge" at all !

Well here the "de" is like "a" but it's more precise. It's THAT limit. I'm sorry it's not really clear but I don't really know how to explain... Good luck.

For my part, I agree with both ;) You can end up your sentence by other terms. For instance : "c'est vrai que nous n'avons pas mis de point à la phrase/n'avons pas parlé de ce sujet/ pas terminé de discuter à ce sujet..." So, in fact, it's just upon the quantity. How many limits have you put at all ? If there isn't, you will add "de". And if there is, you will ask :"c'est vrai que nous avons mis une limite d'âge". I gather that is like your term "any" in somehow way... But the "de" is always something designing a stuff. It's the "de" of the demonstrative.