"How many weeks old is your baby?"
Translation:Quantas semanas tem o seu bebê?
"How old are you?" is phrased as "How many [units] do you have?" in Portuguese.
The correct literal translation for this example is "How many weeks does your baby have?"
The answer would be, "Ele tem cinco semanas" = "He has five weeks" which translated to proper English would be "He is five weeks old"