What is the meaning of aged out for session end reason ?

What is the meaning of aged out for session end reason? 

> show log traffic session-end-reason equal aged-out 

Time                App             From                            Src Port          Source
Rule                Action          To                              Dst Port          Destination
                    Src User        Dst User                        End Reason
====================================================================================================
2018/11/03 18:40:15 ping            L3-Trust                        0                 10.0.8.20
Wide Open           allow           L3-Trust                        0                 1.1.1.2
                                                                    aged-out

• Palo Alto Firewalls
• Session details

1. Aged out session end reason occurs when a session closes due to aging out.
2. For UDP Traffic like Dns traffic is normal to seeing the aged out and it is a normal way for UDP session to end.

3. For the TCP sessions Age out occurs when a session ages out due to timeout.
4. For example if user tries to connect to a server and user is waiting for a response / ack / syn-ack. If the tcp handshake is not established, it times out or firewall did not receive enough packets to recognize the application and it gets timed out.