Our son went to boarding school at 14. I can’t say we noticed much change in our routine home costs during the HS years although we took on a whopping tuition bill. By the time he went to free college, we were in a different house where he lived in the casita when he was home and liked to max the AC. After one crazy bill, we showed him what his freezer was costing us, set the Nest on “eco,” and told him not to touch it from that point on. DH’s company plan paid for our son’s phone in HS, but he took that over when he went to college. The Army also issued him an AMEX card and paid that fee. He was only on our car insurance until the end of his freshman year. That summer, he bought his first car and paid for the vehicle and all related costs himself, so we were spared that. He wasn’t home much during his college years (because Army), so we didn’t notice any significant change in food or utilities.
Apart from the boarding school tuition bill, I can’t say our son’s HS or college absence significantly affected our household expenses, but I’d give anything to have him back home cranking up the AC again. I wouldn’t complain this time.