There must be something about that age ... I am a vegetarian, but didn't make the commitment until I was 40. My granddaughter, however, announced when she was eight that she would no longer eat meat or fish. Her mother was not inclined to be as accomodating as you were. Her response, "Then you can fix your own dinner from now on, because I am not making two meals every night."
And that is precisely what my granddaughter did. Her mother thought that it would be a brief fad--but my granddaughter, now twenty-five, was married last spring--and to this day, remains a vegetarian (although her husband is not). When a child takes a principled stand, a wise parent will respect and encourage it--even when he or she doesn't wholly agree. I commend you for doing so.