What are the legal procedures for "kicking out" your child when they have reached the age of 18?

Well, to be clear, you can’t just “kick out” your adult child.  If rent is being paid (or has been paid), then you may be able to evict as you would any other tenant.  However, doesn’t sound like that’s the situation.  If there is no rent, lease, etc., you may be able to file an unlawful detainer or you may need an ejectment filed depending on the particular facts.  Practically, start by asking him to leave before you file court action.  However, just to reiterate, do no throw out personal belongings, start changing locks, etc. because you could potentially set yourself up for liability.  You say he’s in school… I’m assuming that’s away at college.  If he’s changed his primary residence to the college address then you could be able to get around the legal procedures stated, but I’d be careful.  Hopefully you can work it out amicably as lawsuits among family members are rarely good for the relationship.  Just as a note to the birth certificate and social security card – those belong to him, not you.