It's an adverbial clause, but it's awkwardly worded. The more usual phrase is "when he was of school age."
Stone Age is a noun.
noun
noun
An adjective is a part of speech.
Because it's typically a part of the study of English from grade school through high school.
Irate is an adjective.
The Central School of Speech and Drama is located at 62-64 Eton Avenue, London NW3 3HY, United Kingdom. They are part of the University of London school.
part of speech
The part of speech for this particular word is a noun.
adverb
what part of speech is work
what part of speech is beneath