Do your serial numbers grow with the newer units? If so, you can sort them accordingly to display the oldest on the top.
Depending on how your system is set up, if you have a "Parent" item for all those individual items in inventory, you can summarize the 'smallest' serial number, and display that on the parent. Or use that value to highlight said record.
Its hard to tell exactly what direction you'll need to go, unless you have more information for us about the current application structure and flow.
Cheers,