18-year-olds should not be financially responsible for their parents’ mortgage under any circumstances.

There’s a big difference between an 18-year-old helping their parents pay their rent and putting an 18-year-old’s name on a mortgage that they’ll be responsible for for years. A lease typically only lasts a year or two, so hypothetically, a young person could live with their parents until they’re 20, help pay the rent, and move out once they can afford to take care of themselves. With a mortgage, you expect that you’re going to live in the house for more than a year or two, and you can’t just back out of a mortgage as easily as you can break your lease. All parties on the mortgage have to agree to sell the house before you can get rid of it. With a lease, you can at least work something out with your landlord, like subletting your place or finding a new tenant. But if your name is on a mortgage and the other people on the mortgage refuse to sell the house, you’re still responsible for paying for it, whether you like it or not.