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The only way to be appoint as a legal guardian is via a court order. Here's a good link you can read from the CA Court system's online database: http://www.courts.ca.gov/selfhelp-guardianship.htm
So, you would have to involve the courts in order to get this done......which leads up to your second question about a 19 year old being given legal guardianship, and as you would imagine, the court isn't likely to approve/allow this.