Adolescents understand that all people will die, but often have difficulty to believe that they will die, too. This is comically reflected in our civilization, which understands that other civilizations and species die, but is in denial about its own impending demise.
Probably effects of mass extinction on food chains, poisoning by blue algae and fungi, atmospheric pollution by giant wildfires or so. Perhaps there’s a small chance that AI or nuclear war get us before that :)