In most languages (I know of), people say their age with a construction like I have X years. In English, however, you say how old you are instead. So I'm curious about what is it about the logic of English that makes this sound more natural than I have X years. All I can find is that "this is just cultural/this is just how the language evolved", but I'm sure there's some logic to it.

I'm also particularly interested in the logic that makes the construction I am X years old possible. I suppose it comes from the fact that, in English, we modify/specify words by placing words before the word we want to modify/specify. Something like:

— I am old.
— How old?
— Very old.
— How old exactly?
— 900 years old.

Am I on the right track? Is 900 years in this case really behaving the same as very, or interchangeable with it? (The available information about numerals as adjectives or adverbs is very conflicting.) Or is there something more specific about numbers, or this particular construction, that apply here?