The Second Great Shinobi War started approximately twenty years ago. Sai was then roughly 18. A year into the war, it was completely changed by four men that took over Kumogakure. In less than a year, the new faction had taken over the world and Sai had led it. Sai would be twenty during the beginning of his Reign. Sai's reign lasted just under seventeen years. He was killed at 38. It has been roughly 18 months since Sai was killed, about a year since the Resistance was fully dissolved and The Great Purge has basically ended recently. The current genin have been in the academy about a year.