"Mi hermana tiene diez años."
Translation:My sister is ten years old.
Ages in Spanish are written like that.
La niña tiene cinco años, the girl has 5 years = the girl is 5 years old.
This is a good explanation:
Is Spanish, you wouldnt say "I am (certain # of years) old." You would say "I have (certain # of years." You would use the possesive term "tiene" to show that the person "has" a certain number of years under their belt, so to speak. That's just how it is in Spanish.