It is strange that we say 18 to be too young and stupid to make this type of contract, but not stop them making most other types of contracts. Or signing up for military... If they truly were that stupid, maybe we should significantly increase the age limits for contracts.
This is the point. The military aims to recruit the vulnerable, because hardly anyone in their right mind would sign up to die.
Most contracts won't change your life. They're short-term contracts. Even a car loan is only 4-5 years. Also, the kind of car you drive usually doesn't affect your earnings potential, so going cheaper and buying a used car doesn't hurt you at all. Not to mention, other kinds of loans are not guaranteed by the government, so you can't even get them unless you're credit-worthy, whereas they'll hand out unlimited student loans to any kid.
It takes two sides to sign a contract. Everyone blames the kids, but nobody ever seems to blame those handing out the student loans to people who can't afford the loans. After all, this kind of predatory behavior played a big part in the subprime mortgage crisis.
A few contracts can change your life. A home mortgage, e.g., a 30 year loan. Student loans, which can be almost the equivalent of a home mortgage, in some cases even bigger than a mortgage. Or of course signing up for the military, which could end your life.
18 year olds rarely need to enter into home mortgages. They can rent. Again, renting vs. owning your home usually doesn't have an effect on your earnings potential. And of course 18 year olds rarely even qualify for home mortgages. But society has determined that college degrees are important, and if you don't get one, it can greatly reduce your lifetime earnings. So kids feel extreme pressure to enter into these mortgage-like contracts.