In question types that are made up of more than one word, such as:
"How old", "How tall", "How long" (WH word + adj.), etc.
Do we consider the whole phrase as one question word? Or in all of them, "How" is the only word which is our question word? I don't know if there are any other types, but if so, it would be great to know what kind of grammar I need to study to know more about this structure...
Yes, in terms of sentence structure, "How old" and "What brand of coffee maker" are all considered to be the entire "wh"-word.
It's easy enough to show this by changing the sentences back to statements:
"I am [13]." = I am [how old]. "How old" is the interrogative pronoun that replaces "13", so we move the entire pronoun to the front of the sentence to ask the question.
"I use my [Bodum] most mornings." = I use my [what brand of coffee maker]. Same logic.
Even idiomatic expressions like, "Why/Where/How in the world..." are treated as the whole "wh"-word in terms of word order in wh-questions.
