Mortality is being an adult
Most children don't understand the concept of mortality.
Why should they?
When you are so young and everything is new, you are growing, learning, always expanding your boundaries and never knowing your limits, mortality doesn't fit into your world. It takes experience to grasp the concept that you won't live forever, you won't always survive the bumps and knocks, you'll never get the chance to do everything or to know everything.
There's so much more to the universe than one person can grasp, and to be unable to hold it all, to realise it's not even possible, is to be mortal.