Character ID
Every in-game player Character has a Character ID number. It is automatically assigned by the server upon Character creation. It is unique and sequential, so you may get some idea of how new or old a Character is based on his ID.
You can see any Character's ID number. Alt-Click on a Character and you'll see a short vertical menu. That Character's ID is shown there at the top of the list.
When you Reroll a Character, the original ID is left behind forever, and a new ID is automatically assigned by the system to the new Character.