The answer is: 20

Numbers 1:3

When the Israelites prepared to enter the Promised Land, an important aspect of their community was their readiness to defend and uphold their survival. As noted in Numbers 1:3, only men aged 20 and above were considered eligible for military service. This demarcation reflects a period of maturity and preparation – twenty years is a significant milestone in many ancient cultures, aligning with a readiness to take on adult responsibilities.

Interestingly, while 20 was the warrior cut-off age, the laws and social structures laid out in the Torah offer a fascinating blend of youth and wisdom. Older men, those beyond the age of military service, would often serve as leaders and advisors to the younger warriors. This generational aspect adds depth to understanding communal life in Israelite society, where younger individuals learned from the seasoned members about both warfare and peaceful trade, balancing the thrill of youth and the wisdom of age.

Moreover, it resonates with other ancient civilizations, where enlistment rules often hinged on age and vocational roles, showing us that age played a multifaceted role across societies throughout history. Developing more than just physical capacity, turning twenty could symbolize entering a new phase, marked by the responsibility to defend one's family and culture while deeply respecting the teachings of older generations. So the quest for identity extends beyond mere age, touching upon the fabric of social responsibility that they arguably evoked profoundly within their laws.