"How old are you?"
Translation:¿Cuántos años tienes tú?
No, it is correct, laprica. In English there is no distinction between "you" of tu, usted, or ustedes.
Although less commonly seen, saying "how old are you" for more than one person is valid and can be interpreted as "how old are you [all]", with "all" being understood (much like how "yo" and "tu" are not always needed in Spanish). In addition, the verb "to be" (conjugated here as "are") is conjugated the same way for both singular and plural forms of you.
I hope this helps clarify.
¿tienes cuántos años? could be valid in few certain contexts. If your Spanish region is used to talk like that because reasons or if you were with many things in the head and you gave yourself a pause, ¿tienes... cuántos años?. It makes sense in Spanish and everybody will understand, but it isn't a recommended construction.
In early lessons we had to put e.g. Yo tengo then later tengo was enough. I thought that Tienes was the correct form of to have for the pronoun tu, which should therefore be redundant I'd no need for th tienes. Where am I you by wrong? I can't believe the question was directed at a group of people.
Haha, for babies, you generally ask their parents with the followings: ¿cuántos tiene? (here you're dropping meses because it's clear), ¿cuántos meses tiene?, ¿cuánto tiempo tiene?.
In addition, translating How old are you you get ¿cuán viejo eres? which is kind of offensive, specially to not-so-young people. ¿Cuántos años tienes? or ¿qué edad tienes? are the most accepted and polite forms.
While there could totally be nerdy kids who want ask each other how many days/weeks/months old they are, or there could be situations in math class where it is necessary to calculate how many days/weeks/months old someone is, those would need to be phrased as "How old are you/is so and so in days/weeks/months?" to show that someone wants to see age expressed in some unit of time other than years.
In Spanish they never literally ask "How old are you?" they ask "¿Cuántos años tienes?" which means "How many years do you have?", and you answer "Tengo ? años." It would of been nice if they offered this in the hints though because even knowing this I entered the ultra literal translation I've become accustomed to DuoLingo wanting.
I'm not certain on this. But I think of viejo as old, an adjective, and does not mean age, a noun. Ser describes something more permanent about a person, and age is constantly changing. would qué estás tu viejo? Be acceptable maybe... :/ ? I think in this case you just memorize a person's 'age' as 'having years'.