Learn Dutch: Naarmate een kind opgroeit, verandert het brein. - As a child grows up, the brain changes.

Naarmate (as/in proportion as) is a subordinating conjunction. In Dutch, these conjunctions push the working verb to the end of their clause, which is why opgroeit sits at the very end of that clause, right before the comma.

Why isn't the separable verb opgroeien split into groeit ... op here?

Separable verbs usually split when they sit in the standard second position of a main clause. However, because the subordinating conjunction naarmate forces the verb to the end of the clause, the prefix (op) and the conjugated verb (groeit) are pushed together and merge back into a single word: opgroeit.

Why does the second part of the sentence start with the verb verandert instead of the subject het brein?

This is inversion caused by the V2 (verb second) rule. The entire subordinate clause (Naarmate een kind opgroeit) acts as the first informational block of the sentence. To keep the main verb in the second position overall, verandert must come immediately after the comma, swapping places with its subject (het brein).

Does naarmate mean the exact same thing as als (when) or terwijl (while)?