This past week the Supreme Court heard arguments about whether children should ever be sentenced to life without parole for crimes that don’t involve murder.

At the heart of the argument lies a vexing question: When should a person be treated as an adult?

The answer, generally, is 18 — the age when the United States, and the rest of the world, considers young people capable of accepting responsibility for their actions. But there are countless deviations from this benchmark, both around the world (the bar mitzvah, for instance), and within the United States.

For drinking, driving, fighting in the military, compulsory schooling, watching an R-rated movie, consenting to sex, getting married, having an abortion or even being responsible for your own finances, the dawn of adulthood in America is all over the place.

And if you think separating the men from the boys (or the women from the girls) is difficult today, tracing the history of America’s conception of childhood just complicates things further.