If animals are here for us to eat, ask your mom when the last time she ate a Grizzly bear was.
Or, you could tell her she has a very anthropocentric view of animals.
Incidentally, I was 15 or 16 when I suddenly turned vegetarian, my mom just kind of rolled her eyes and dealt with it. At 18, you are old enough to choose what is right for you. If your mother makes snide/rude/ignorant comments just ignore those comments, easier said than done, I know.