No reputable coven that I know will take anyone who is not legally an adult without at least having written permission from their parents. It is against the law for anyone to "interfere in the religious upbringing of a minor" without the consent of the parents. Should a coven take you in and your family objects the members of that coven could be heavily fined or even sent to jail.
However, that certainly doesn't preclude trying to find a group near you so that you are ready to approach them when you turn 18.
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