"I was four years old when I went to Norway."
Translation:Jag var fyra år gammal när jag åkte till Norge.
The idea of "switching" is not helpful. The thing is this: in main clauses, the verb goes in second place in the sentence. In subclauses, the subject goes before the verb.
The main clause is Jag var fyra år … – here, the verb is in second place.
The subclause is när jag åkte till Norge – here, the subject is before the verb.
Long post about Swedish word order here: https://www.duolingo.com/comment/8970470
Can you construct a similar sentence in which the inversion of subject and verb does occur? For example, "Nar jag var fyra ar, akte jag till Norge."
So this changes the first half into the subclause (subject first) and that entire phrase fills position 1 so the verb "akte" comes next?
I don't mean to suggest this should be an accepted answer, I am just checking to see if I understand the word order rules correctly.
If you think about it like this: I was four years old - I was how old - I was old – you can see that old modifies I. So since you'd say Jag var gammal, you also say Jag var fyra år gammal.
But if we're speaking about an ett word: Huset är gammalt 'The house is old' – the adjective is in the -t form.