The answer is: 20

Numbers 1:3

A fascinating aspect to the age of 20 as the starting point for Israelis going to war is that it branches off from a rich symbolic background found throughout the Hebrew Scriptures. This age represents a significant transition from youth to adulthood. Among many ancient cultures, turning 20 was often seen as the completion of one’s passage into full maturity. In this way, the Israelites set a clear standard—after 20, a person would carry the full weight and responsibilities associated with adult life, including military service.

If you delve a bit deeper, Numbers not only covers the census of eligible warriors but essentially reminds us of the greater mission of community and accountability that lay deeply rooted within Hebrew society—this wasn’t just about warfare, but about the very identity and survival of the community. Speaking of identity, the Hebrew Scriptures often echo themes of endurance and communal responsibility, drawing striking parallels with later texts, where individual and collective identity hold camaraderie during challenging times.

Additionally, interestingly, this law echoes stories from heroic elements in other cultures—the Greek hoplites, for instance, were also of a similar age bracket, further stressing how societies throughout history have linked maturity with military service. Just food for thought, this idea carries through to modern contexts where we continue to nominate an age for responsible engagement in society, proving the impact these historical laws have wrought through the ages.