A "Das ist/sind"-sentence is a so-called "Gleichsetzungssatz". A simple sort of "Gleichsetzungssatz" is:
Meine Frau ist Pilotin.
Or simplified: X is Y.
If X or Y is a "Demonstrativpronomen", it is - regardless of its grammatical gender or number - the demonstrative pronoun das.
This is similar to English where you can answer the question "To whom belongs this book?" with "This/That is my book."
It doesn't matter if the question is phrased in a genitive or dative structure, you can always answer the question with "Das ist/sind"
Wessen Bücher sind das? - Das sind meine Bücher.
Wem gehören die Bücher? - Das sind meine Bücher.
Von wem sind die Bücher? - Das sind meine Bücher.
Sind das deine Bücher? - Ja, das sind meine Bücher.
As you see in the first and last question you also use the "ist/sind das" structure.
Of course you can answer any of these question in a different way:
Die Bücher gehören mir.
Die Bücher sind von mir.
So, again: In a "Gleichsetzungssatz" you can use the "Das ist/sein" structure and "das" can refer to any grammatical gender and even to plural nouns.
You can replace "mein Buch" or "meine Bücher" with "mein(e)s" or "meine", respectively. This does not affect the rule mentioned above.
If you do not use a "Gleichsetzungssatz" as in your second example you must use the proper pronoun:
Der Film ist ab 18.
Das sind meine.
is a bad answer toWem gehören diese Bücher?
but you will here this answer a lot in spoken language. Stylistically better would be(Die gehören) mir.
. If you want to keep the answer, you should change the question toWas sind das für Bücher?
.