I told my parents that when I was a kid, too. I've been vegetarian for 30+ years now.
My daughter was about 12 when she became a vegetarian. She didn't give any reason, she just stated that she would no longer be eating meat. My response was, fine, but I'm not cooking anything special for you. So, she ate the veggies and didn't eat the meat. We went to the pediatrician and he said give her a multi-vitamin with iron and peanut butter and eggs (if she's willing).
We thought it was a phase that would last until we cooked bacon on Sunday morning. That was over twenty years ago, and she still doesn't eat meat. I taught her to not be rude when she went to other people's houses or restaurants and just eat the veggies, bread, pasta. She can always find something that she enjoys even when we go to eat barbecue.