Honestly, spending so much time studying space with my son is a trigger for my existential depression. I have to guard against it, and I'm sure it's all part of why I'm in language arts. Anyway, he will ask about the world ending but luckily in a theoretical way so far. My daughter, on the other hand, has begun expressing a very "nothing gold can stay" sort of depression, and it is heartbreaking to see this in a seven year old.