Because the spanish say ' the girl has nine years'. In English that means she is nine years old. So if I were telling someone I am 35 years old (I wish) I would say 'tengo 35 años' which in spanish says I have 35 years. Hope that helps explain.
If someone didn't know how Spanish works then this could sound very intimidating.
I mean, that's the literal translation, but it's not really what the phrase means in English.