Why does nobody take my vegetarianism seriously?

About a month ago, I decided to become a vegetarian. It wasn't something impulsive though, I was thinking about it for a long time. I finally took the plunge properly once I hit 16 but ever since I began telling people, they treat the subject like it is some stupid teenage phase I am going through. But it isn't, it is something I feel pionately about and I plan to stay this way as long as I live. I still even get prepared meat for my meals and people say that it's bad for my health and it is a stupid and pointless thing I am doing to myself because it won't save any animals. Has anyone has had this problem and what could I do to show I am serious?