The definition of resident is different for different purposes. For instance, for state tax purposes, you’re most likely a resident of California. California wants to make sure it’s collecting taxes at in-state rates from everyone who lives in the state and enjoys the benefits provided by the state. For in-state tuition for college, it wants to make sure that the relevant responsible, tax paying adults have made a real commitment to the state. No sending your kid to live with his aunt in California for the senior year of high school to get in-state rates. No claiming resident status after finishing freshman year. So the state has rules about who counts as a resident. Can someone be raised by someone other than their parent? Sure. And I’ll bet the rules cover those instances. But it appears you were raised by your mother, since you moved to Seattle with her and stayed until you were 18. I would bet that if, when your mother moved to Seattle, she left you with family in California and transferred legal guardianship of you to your grandmother or whomever, you might well qualify as a California resident (NB, I’m making an educated guess. I don’t actually know the rules about that.)
People under 18 years old generally have someone who is their guardian. Your mother was your guardian. Yes, other people may have helped raise you, but your mother was the person legally responsible for your well being and entitled to make decisions about you. Once you hit 18, you became a legal adult and no longer had or needed a legal guardian.
The reason for the rules about people between 18 and 24 is to reduce the extent of people gaming the system.