She says "E' cinque" in the audio.
Still says 'è cinque' on audio
Don't get why "at age five" is ok but "when he was five" is not...
Because to directly translate to that, Italians would say "Quando (lui) aveva cinque anni...".
Di chi Duo sta parlando? Di Mozart?
you should say: when 5 years old or at five years (of age)
Both of those would be incorrect in English. "At five years of age" or "when he was five years old" would at least be grammatically correct English, but neither is a proper translation of the Italian sentence.