Three Star retired General Humphrey Njoroge in his book Promises to Keep and Miles to Go has explained how military appointments are done.
The General said the Defence Council is the ultimate for all rules, procedures and instructions concerning the KDF.
"These involve criteria for promotion, appointments, and continuous training. Criteria for enlistment, commissioning, promotion from one level to another are also within the purview," the general said
He said the entry age for cadet recruitment is a minimum of 18 years.
Other criteria include medical and physical fitness and educational qualifications.
He said Lieutenants recruited at 18 years should attain the rank of Captain by 37 years and failure to do so they exit the army.
"Captains are promoted to Major depending on their performance or vacancies available," he said.
The General said Majors who reach 42 years before promotion to Lieutenant Colonel automatically retire.
He said Lieutenant Colonels retire at 45 years.
"If they reach that age without further promotion to full Colonel, they exit. If Full Colonels are not promoted to Brigadier by 48 years, they too exit the military," he said.
The General said Brigadiers serve until 52 years, and if not promoted to Major General by then, they too exit the military.
He said Major Generals retire at 54 years.
The Army Service Commander, who is a Lieutenant General, serves for four years or until 58 years. Whichever comes earlier.
He said Lieutenant Generals retire at 58 years with no exception, except for the one promoted to a full General-CDF who retires at 62 or four years in appointment, whichever comes earlier.
The General said these rules are referred to as the Tonje rules.
General Daudi Rerimoi arap Chepkonga Tonje was the Chief of General Staff between 1996 and 2001.
General Tonje was also the Commander of the Kenyan army between 1993 and 1996.
He persuaded the Defence Council to adopt the rules and follow them.
"The reason why service commanders retire at the age requirement of their rank or 4 years in command whichever comes earlier, is to ensure that they do not become individual institutions," the General said.
Thus, even if appointed too early, CDFs do not serve for more than four years.
He said those promoted too late - towards 58 years- serve only for the remaining period until they reach 58 years.
Navy and Air Force Commanders, who are Major Generals, retire at 54 years or four years in command, whichever comes earlier.
General Tonje was a visionary General in the Kenya military.