"This girl is five years old."
Translation:ʻElima makahiki o kēia kaikamahine.
So in English we can say, "this girl". That is a demostrative adjective - it is attached to a noun. Then we can also just say "this", like "This is milk." That is a demonstration pronoun - it is standing alone and representing the whole noun.
Same thing in Hawaiian. "Waiū hou" means "fresh milk". "Kēia maukuahine" means "this girl". "Hou kēia waiū," would mean, "this milk is fresh." You can tell kēia is attached to the milk because it is in front of it. If Kēia is after the milk, then it must be doing something else, so, "He waiū hou kēia," means, "This is fresh milk," and the kēia is acting all on its own and not attached to anything.
That makes a bit more sense if you know of other languages (like Spanish or French) where your age is the number of years you have, not the number of years you are.
Yo tengo catorce años. J'ai cinquante ans.
Not really a concept that carries over to English or, for example, German.
I am thirty. Ich bin fünfzehn Jahre alt.