Is it common in Norwegian to discuss age in terms of decades and centuries rather than numerical values, or is this simply due to the lesson structure?
Simply due to the the lesson wanting to teach the word 'tiår'. If you wanted to express the age of a young boy, you would say "Han er ti år gammel" = "He is ten years old".
Is there any reason why this would not accept "He is a ten year old"? From the sentence structure it almost implies we are talking about "a ten year old" as a thing that he is rather than simply him simply BEING 10.