the 42 day age is the age since the last sucessful password change. So to answer you question. No they will not all expire in 42 days. You could use
modprpw -E
This option is specified WITHOUT a user name to expire all user's
passwords. It goes through the protected password database and
zeroes the successful change time of all users. The result is
all users will need to enter a new password at their next login.