Is it possible for a 15-year-old to move into another household with a family member at the age of 25-year-old or 26-year-old if life at home and school is getting unbearable?

It is possible, with parental consent, for a child to live in almost any situation as long as it is not dangerous. Without parental consent, it may be possible, but parents could demand that the child return home at any time. Other considerations are: for enrolling in school, consent to medial procedures, and other practical purposes, a family member cannot act as a parent unless they have proof of custody. Even if parents don't object to the situation, if they are not actively supportive it may be difficult for the 15-year-old to enroll in school, get permission slips signed, visit a doctor, etc. Laws vary from state-to-state, consult local resources (such as non-profit legal aide orgazations or state government websites) for more information specific to your region.