This field is the age of the link state advertisement in seconds. It should be processed as an unsigned 16-bit integer. It is set to 0 when the link state advertisement is originated. It must be incremented by InfTransDelay on every hop of the flooding procedure. Link state advertisements are also aged as they are held in each router's database.
The age of a link state advertisement is never incremented past MaxAge. Advertisements having age MaxAge are not used in the routing table calculation. When an advertisement's age first reaches MaxAge, it is reflooded. A link state advertisement of age MaxAge is finally flushed from the database when it is no longer needed to ensure database synchronization. For more information on the aging of link state advertisements, consult Section 14.
The LS age field is examined when a router receives two instances of a link state advertisement, both having identical LS sequence numbers and LS checksums. An instance of age MaxAge is then always accepted as most recent; this allows old advertisements to be flushed quickly from the routing domain. Otherwise, if the ages differ by more than MaxAgeDiff, the instance having the smaller age is accepted as most recent. See Section 13.1 for more details.