Numbers 4:21

but thus do to them, that they may live, and not die, when they approach to the most holy things: Aa...

but they shall not go in to see the sanctuary even for a moment, lest they die."

"Take a census of the sons of Gershon also, by their fathers' houses, by their families;

you shall count them from thirty years old and upward until fifty years old; all who enter in to wai...