Try coaching your passengers to back down the step. That way the passenger will not "drop" all of their weight on the step but put it on gently, which should reduce the stress on the step.... I think the difference is the exuberence of Young Eagle passengers at times on the passenger side (lighter weight of YE passengers is less of a factor than kid's tendency to "drop" weight onto step even after coaching not to), along with occasional "heavier than FAA standard" passengers. I've looked at a couple -9As locally with same thing - passenger side cracked and pilot side holding up well.