Final answer:
Spartan men served in the military for 20 years, starting from the age of 30.
Step-by-step explanation:
Spartan men served in the military starting from the age of 30 for 20 years. This period of service was known as the Krypteia. During this time, the men would continue to live in their military barracks and engage in military training and service. After the age of 50, Spartan men were allowed to retire from active duty but still remained part of the Spartan military reserves.
Learn more about Spartan military service