"This girl is nine years old."
Translation:Esta niña tiene nueve años.
Even in French we do say (j'ai neuf ans, which mean I have nine years litteraly but means I'm nine in English. ) French and Spanish use the same rule and verb ( To have) to tell age instead of ( to be) in English
Why is this "tiene nueve años" and not "esta nueve años" or something that uses the word "is" and not "has"
Because in Spanish you say How many years do you have? I have 9 years. In Spanish that is the same as How old are you? I am 9 years old, in English. But it would not be correct to say I am 9 years old in Spanish, just the same as it would not be correct to say I have 9 years. . . in English. Hope that makes sense
To add to what poefae said, there are a lot of expressions in Spanish where you have something, which you would be in English. Examples include having hunger (tener hambre), having thirst (tener sed), having tiredness (tener sueño), having heat (tener calor), having cold (tener frío), having pain (tener dolor), etc.