When I was 10 I rode a cardboard box down the stairs
Why would you do such a thing?
I thought that it would be more fun than riding the elevator.
It's okay to be a little reckless, and to do some dumb things for fun when you are a kid, but this was risky behavior.  It sounds like you were in public when you did it, which is even worse.  Elevators are extremely safe, safer even than stairs, so you should not be afraid to use them.
Nobody got hurt so it wasn't a big deal.
Even so, it doesn't mean there wasn't a chance that people would get hurt. As long as you learned from it and made smarter decisions going forward, it is fine.