I'm so scared that my parents are going to die. How do I learn to just accept the fact and prepare myself for the moment?

It does happen. That’s life. Happend to mine. It will happen to yours, you, and your kids. It will happen to everyone.
Just go out and celebrate life. Don’t worry about the downside, which is if you live, you’ll die. Would you rather have been a stone and not lived at all, or a human who lived, loved and had fun, but then died?