That is relevant but does nothing to explain why say Sweden put the line at 16 while the US has it at 18. It also happens to be the exact opposite for driving a

That is relevant but does nothing to explain why say Sweden put the line at 16 while the US has it at 18. It also happens to be the exact opposite for driving a car, ie the age requirement is 18 while in the US it is 16. Personally I find the age when the brain is fully developed to carry a bit more significance, ie around age 25. Before that the brain is still being assembled and yet we expect people to make life or death decisions while driving, serving the military, have correct impulse control and follow the law just as everyone else.