No native speaker would ever ask someone's age this way, it really sounds very odd in Italian.
The only common way of asking another person's age is:
- (Lui) quanti anni ha? = (literally) How many years does he have?
Only speaking of a baby, usually people ask:
- (Lui) che età ha? = What age does he have?
as the reply would be expressed in weeks or months, rather than in years. But it would not be used for a child above the age of 1.
Another informal way of enquiring about a baby's age is:
- (Lui) quanto ha? = (literally) How much does he have?
referring to 'time' (i.e. age).
But it is enough for Duolingo to teach the standard way of asking one's age, which is always based on "having years", rather than "being years old".