"Halo 4: I Get a Woman!" - Sgt. Mjr. A.J. Johnson, RIP

[Tnnaas]

Er. wat.
Halo 4 takes place maybe a year or two after Halo 3 and Halo 3 was only 1 year after Halo CE.
The original Halo was set place in 2552 and Cortana was "created" just before halo CE[months if not weeks].

Cortana was created well be fore the events of Combat Evolved. By the end of Halo 3, Cortana is seven years old and already diverging into rampancy with plenty of help from both the Forerunners and the Gravemind. Halo 4 is said to take place five years after Halo 3. Simply basing my observation on other reports of seven-year-old AI within the Halo canon, they are often terminated by that time by ONI or they accidently kill themsevles while trying to keep themselves out of rampancy. Smart-AI, at least.

Fixed that for you. And AI don't "die" after 7 years, but rather become self-aware and divulge into rampancy.

Still, twelve years is still a long life for an AI. So, we are either going to deal with a rampant bitch this trilogy or she's found a way to tamper with the Dawn's FTL drive and shunt most of her memory into 11-dimension slipstream space to avoid said rampancy.