In the US, 18 years of age is generally accepted as the age at which one may choose to serve (and risk life) in service of country.
And of course, today we are witnessing the infantilization of people who are that age – and much older. Large numbers of young people are simply not launching. We cannot even remove a kid from our health care plan until they are 27!
I thought it might be interesting to reflect that, although there is a tradition of Judaism (the Bar/Bat Mitzvah) that tells a person they are responsible for their actions from the ages of 13 or 12 (roughly approximating puberty), this does not come from the Torah!
Instead, the age of maturity in the Torah, for national or priestly service or to be counted for the census, or even to be considered responsible for our actions (during the spies), is twenty years old and upward.
So why is 20 the age of maturity in the Torah? I think it is simply because, in every example where the age of “20” is given, it is connected to investing in other people. Until then, people are too interested in themselves to realize that connecting to others is essential part of being a member of society. It is a standard descriptor of children, and certainly of teenagers (in my experience).
Jacob marks 20 years of investing in Lavan:
This twenty years have I been with thee; thy ewes and thy she goats have not cast their young, and the rams of thy flock have I not eaten. … Thus have I been twenty years in thy house; I served thee fourteen years for thy two daughters, and six years for thy cattle.
The number 20 is used when Jacob reaches out to attempt to invest back into Esau: He gives (among other things), twenty each of he-goats, rams, and she-asses.
The number 20 is also used for the boards of the tabernacle – 20 for each length – forming the critical enclosure that enables G-d’s presence to dwell among – to invest in – the people.
The conclusion we reach suggests that G-d basically writes off the first 20 years of a person’s life as an opportunity to learn, but without direct, serious consequences (which is why those who were younger than 20 at the crisis of the spies were not held responsible). So then a full life of 120 has 20 years of preparation, and 100 years of fullness. Which in turn corresponds to the way “100” is used in the text for completion: e.g. the length of Noah’s Ark, the tabernacle, and the lifespan of Moses.
P.S. The value placed on a 17 year-old: [they] sold Yosef to the Ishmaelim for twenty pieces of silver, is the same value assigned for legal value: And if it be from five years old to twenty years old, then the estimation shall be of the male twenty shekels. Did Joseph’s price set the value for later in the Torah?