That is the age requirement for a child to sleep on the top bunk of a set of bunk beds.
I now know why.
It is not a safety issue.
It is a matter youthfulness and flexibility. Six year olds are excellent climbers. Six year olds are very capable of making their own beds. They have to be. No one else is physically able to climb to the top and make the bed. Take it from me. I know.