I don't understand why the answer is "ustedes." The speaker seems to be talking to a single child which would suggest singular, familiar and not formal and plural. Help me understand why.
8 Esa película no es apta para ________. Solo tienen 10 años.That film is not appropriate for you. You are only 10 years old.
(HINT: Use the right pronoun after the preposition.)
nos
ustedes
sus
ti