Sam Neill's character mentioned he had cancer, was dying, and didn't know how to tell his daughter they had limited time together. He got infected and said now they had all the time in the world. It sounds like the disease cured his cancer. He probably wasn't the only one. I can see how people faced with a terminal disease are suddenly cured and feel immortal from it. Maybe there were other mortality benefits, such as getting hurt and healing quickly. The brother got burned bad in the face and then seconds later look perfectly fine.
The teen girl in the beginning killed herself because she was not getting any older. Adults might not change much in ten years but clearly someone like a teen should have some changes in ten years. If they were twelve when they changed and still look twelve ten years later, people might think they're ageless.
Just look at it this way; if no one died from natural causes in ten years and no body aged, we would think we were immortal as well(at least until someone died, then we'd change the classification).
they saw other vampire movies and guess the stuff 2 plus 2 Well, actually, "kids do not age" and "no single death in 10 years" answers work perfectly Born2Burn