It's not so much that he's traveling until he turns 18, but that you give consent until he turns 18. If that part is even necessary, I'd place it at the beginning of the sentence. It might not be necessary because after he turns 18, your consent isn't required.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
Until my son, Jimmy Smith, reache