To display the ages of the message currently on the queue Use the following options on the command line:
dmpmqmsg -m QM1 -i Q1 -f stdout -dT
This is very use full In testing scenario where you want see the message age that were not read by the application .
you can use other dmpmqmsg commands to dump to another file , read out ,delete etc .
Refer below link for more details
How to delete/backup/restore old/new message from a queue in ibm mq