MAC address table does age out (after 600 secs) I think the 'lastseen' column is intended as a long term guide to when & where devices were attached to the network rather than an up to the minute indicator. So if you look at 'lastseen' it tells you when a device disappeared off the network only accurate to the frequency of the scans.
The CatOS equivalent of 'sh mac- dyn' is 'sh cam dyn'.
If this is a long term project, then you really need to be using SNMP rather than CLI. There is an SNMP object which will tell you how long since a port status last changed (went from connected to unconnected or vice-versa). That would be a better way of checking if devices are still connected because it is less CPU intensive than repeatedly downloading the MAC address table to check if a device is still connected.
