If you want to mention the age of a person, we say "Mary is a young girl aged 13", i.e. "aged" appears right after the person without "be--is". It seems that many online examples use "aged" in this way too. I seldom see examples like "he is a boy who IS aged 14". I wonder:
1) is it ungrammatical to put "is" right before "aged" even it is an adjective telling sb's age?
2) if it is not incorrect, then is "be+aged" uncommon among native speakers?
Re: aged/is aged
This is just about OK, but it is shorter and neater without the additional words you have added- aged can function as an adjective.
he is a boy who IS aged 14
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