As far as I can tell, Duo has not addressed the formal "you." For a beginner, and/or someone who does not know romance languages, this could be a new concept. Duo marks you wrong if you translate "you" when a 3d person, sing. verb is used. I tested this and lost a heat. I wonder why this has been omitted.
Yet another example of D/L being out of touch with english idioms. It is permissible and generally accepted for one to say a person has so many years( (of age) is understood) just as it is permissible to say that they are about a certain age. Please learn these english as opposed to american usages.