I think adding the expansion to include the 16-18 years is a great idea. For example, looking back at the show Stranger Things, the older kids had an immensely different adventure than the younger ones. Different goals, different Troubles, harder to find Anchors & Relationships (older teens tend to be more loners). Im a GM that loves a good overall story arc, and taking the group from running around solving mysteries (what is old man Smithers doing at school late at night) to facing off against more menacing foes (old man Smithers is feeding teachers to a strange plant in the school basement!!) is what I love to do.
The mechanics are there.. and they are so simplistically beautiful. I think it would just need to make a few adjustments to help PCs and GMs alike. As the PCs move out of the wonderment of the 10-15 age, they lose their luck (LOVE that btw).. would you then get something like negative luck points, "bad luck" as the older teens? Perhaps utilized as a dice pool for the GM - or to take away successes at key points in the story to represent just how hard life can be - or at least how hard older teenagers feel life can be. (Tommy had made that jump a thousand times, he could do it in his sleep. Just cut right down the alley, then up and over the fence, and he would be home free. (1 success, but he GM gives bad luck) His foot slipped, his sneaker got stuck in the chain link fence and he crashed hard on the pavement below. As Tommy wiped away the blood from his nose, he looked up into the faces of the jocks that had been chasing him. This was about to get ugly.)