"Graduating from the Academy during the Third Shinobi World War, Shisui was placed on a genin team alongside his best friend. Immediately after the war ended a year later, Shisui witnessed his friend die on a mission, consequently awakening the Mangekyō Sharingan as he felt responsible. A year later, he met a 5 year old Itachi Uchiha"
Shisui died at Age 14-16 (General Consensus being 16), with Itachi being 12-13 at the time.
He met Itachi when Itachi was 5 Years Old, Making Shisui Between 6 and 8 years old.
Meaning Shisui Should've graduated around age 5-7
putting him on the top shinobi to graduate, amongst others such as Kakashi Hatake, Might Guy, The Sannin, etc...