This object reports the serial number of the oldest route currently in this autonomous system's topology table, part of EIGRP's internal sequencing scheme where every inserted or deleted route gets an incrementing serial number starting from 1, with 0 meaning the topology table is empty. An admin doesn't typically tune this directly, but the gap between this value and cEigrpNextSerial gives a rough sense of how much churn, inserts and deletes, has occurred in this AS's topology table over its lifetime relative to how many routes are currently retained. A topology table where this object is 1 and cEigrpNextSerial is in the tens of thousands would tell an admin this AS has experienced substantial route churn over its lifetime even if the current route count looks modest.