Help for the MsgAge filed states:
The length of time in seconds that the oldest
message has been on the queue.
By DEFAULT the EXTENDED option is set off.
This option must be ENABLED to gather this information.
This option can be set on the MQQPERF display under the Options.
See note in the Notes section for additional information on this field.
Note 3 states: IBM MQ only gathers this information if monitoring is turned on for the queue.
The queue's MONQ parameter must be set to LOW, MEDIUM, or HIGH.
To display or alter the value for a queue's MONQ parameter, use the MQQLOCAL or MQALTER commands.
You can also select the queue from MQQPERF or MQQLOCAL to navigate to the MQALTER command for the queue.
To display or alter the value for the queue manager's MONQ parameter, use the MQMGR command.