The Spartan tactic of using territorial reinforcements was a necessity but at the same time a trap which limited the size of the army. Because the Perioikoi were more numerous, they offered a large auxiliary force, but their numbers in battle had to be limited by ratio to the number of Spartiates, otherwise the balance of the army’s power would be held by outsiders. This requirement led to the unfortunate consequence of a decrease in the size of the army as the number of available Spartiates declined.
Spartans had forty years of eligibility for service, from age twenty to sixty. To prepare for war, they chose a call up age limit based on the number of hoplites needed -- sometimes age forty-five, sometimes fifty, and in extreme cases, more.