Are parents of 19 year old responsible after the 19 year old signs a lease with boyfriend and then 19 year old wants to move out?

In the U.S.A., the age of legal adulthood is 18. So, if a 19 year old signs any kind of legal contract, it is binding and the sole responsibility of that 19 year old. The only instance I can think of that would be different is if the person in question (the 19 year old) was, for some reason, still the legal responsibility of his/her parents. For instance, if the 19 year old was mentally challenged in some way and his/her parents were the legal guardians because the person was too disabled to make their own decisions.