Para preguntar:
How old are you? - Qu edad tienes?
Para contestar:
I am 20 years old - Tengo 20 aos
I am 20 (forma corta)

Se emplea nicamente el verbo TO BE

I am ... years old NO I have ... years old.
Se usa la forma corta o la frmula "years old"
- She is 35 / She is 35 years old NO She is 35 years.

La edad de los bebs suele expresarse en meses.
- The baby is 3 months old.