As the film makers studied lions, and in the film and on the Laserdisc release bonus material they distinguish appearence between a cub, adolescent and adult--we can safely say they had the characters age basically according to real life.

Thus, cub Simba couldn't have been much older than 7 months old when he was exiled, and in the first adult Simba scene he has a fully grown mane, which makes him at least five years old going on six. This results in approximately four years in exile--but on the commentary track the film makers say that after the first adult Simba scene, at the destroyed Pridelands scene, they intented to have subtitles "Kenya, 2 years later". This adds two years to Simba's exile.

Simba's exile lasted approximately 6 years, making him approximately 7 years old when he reclaimed the throne.