Is the question of "what age are you" idiomatic,common or does it
sound naturally in the meaning of " how old are you "
Can we say this structure could be used for materials as " how age is
it "
"What age are you?" is definitely not idiomatic. Using it would mark you instantly as a non-native speaker. However, it makes logical sense and it would be understood.
"How age is it?" would be even worse. While most native speakers could likely figure out what this meant, it sounds simply wrong.
If you were asking a person about his or her age, you'd almost always say "How old are you?" or "What year were you born?" or "When were you born?" -- with the first one being by far the most common.
If you were asking about an object, you would usually say "How old is it?" But you might also say "When was it built?" (in reference to a building or structure) or "How long has it been here?" or "How many years has it been here?" or "How long has it been around?" -- again, the simple "How old is it?" would be most common.
The main difference between these is that these phrases contain only one thing: the subject - my book(s) - and so it takes the nominative case (although there is usually no difference between cases in English).
Do you know where my book is?
Do you know where my books are?
Whereas the phrase:
Do you know how many feet are in one mile?
contains two things: subject and object - feet and mile.
Asking the main question by itself for each case would be written:
Where is my book?
Where are my books?
How many feet are in one mile?
In each case the phrase is led by a Question Word (Where, How, What, Why, Who, When, etc.).
When these questions are rewritten with a main clause and subordinate clause, the formation can change depending on the construction.
You can consider that the original question is being constructed from two separate questions. i.e.
Where is my book? Do you know?
Where are my books? Do you know?
to
Do you know(,) where my book is?
Do you know(,) where my books are?
NB: The comma in parentheses (,) denotes the separate clauses.
In this construction, the (conjugated) verb goes to the end of the clause.
In questions with subject and object, we usually don't change the word order when we combine the clauses.
How many feet are in one mile? Do you know?
becomes
Do you know(,) how many feet are in one mile?
The formulation
Do you know how many feet one mile is?
may be heard in spoken English but this is grammatically incorrect, as is
How many feet one mile is?
Best Answer
It is indeed informal (I don't use the word slang).
Your paraphrase is grammatical, but I don't think it has the same meaning. I don't think if is what's omitted.
I think the full version would read something like:
A more normal version would be