No, Daniel. Donay got it right, that´s "how old are you?"
And that answer could be "twelve" (informal), but being a shortening for "I am twelve'' or still better "I am twelve years old."
Brazilians incur in this particular mistake all the time, because we think in terms of "numbers" in Portuguese (eu tenho doze...)
Whereas in English, the reasoning (focus - if you wish) is all about time, "I am...old".
Even babies are X months old:http://celebritybabies.people.com/2015/ ... odie-baby/Her daughter Ryan Ava Erhard is only 4 months old, but Haylie Duff has already achieved an important parenting milestone.
Sometimes you can find "she is/was 27 years of age" (not everyday, more commom to see "she is aged 27 or 27 y.o.) but it´s not wrong.
To say that someone has...age
is a no no in English!