En los EE.UU. la forma normal es "How old are you?"
Tenga cuidar, la pregunta se considera inapropiado con extraños. Es una materia personal. Entre amigos, es apropiado, y por supuesto si una persona menciona que es el día de su cumpleaños, debe anticipar esta pregunta. Si una persona evita la pregunta, o hace una broma--por ejemplo, una mujer de cincuenta años diciendo "mas de veinte y uno," no diga más.
In the United States, the normal form is "How old are you?"
Be careful, the question is considered inappropriate with strangers. It's a personal matter. Between friends it is OK, and of course if a person mentions that it is their birthday, they should expect such a question. If a person avoids the question, or makes a joke--like a fifty-year-old woman saying "over twenty-one"--don't say anything more.