I think teenagers have the "realisation" if that makes sense. My younger SIL went through similar stuff when she was a teen, but it didn't last. Whereas my kids godfather had the realisation as a teen and is still a strict vegetarian and animal rights activist to this day. So it really depends on the person.

Just make sure whatever you do that everyone respects her for her decision, vegie angst as... when vegetarians/vegans are mocked for their choices can lead to the assumption that all meat eaters are this obnoxious or naive. And it creates an unnecessary division. I know for my kids godfather he suffered a lot of flack for his choices from friends and family and it kind of tainted his view of those who ate meat. But he's fine now and I guess it is a lot more socially respected than it was 20 odd years ago.