I will be honest in the past I skimmed over the book of Numbers , but today I ask God to help me learn from this book.
He answered in the third verse , 3 from twenty years old and above—all who are able to go to war in Israel. You and Aaron shall number them by their armies.

God is a God of numbers He is our Great Mathematician, If you ever had questions of when God considers us adults I guess that verse would help answer that question. God only sent males into battle as soldiers . We as a country held to that same command until the 1990s. Much has changed in a short time in America and this is just one of them .