It is all arbitrary though. There are some weak arguments about mental development but other than the generally accepted rule that human brains stop developing at around 25 years old, there isn't any hard science between 16, 18, 21, or whatever. Individuals hit developmental milestones at different ages, whether they are physical or mental. Each age-restricted activity requires different types of development. A high schooler may be able to make an informed decision on who or what to vote for, but will be subject to peer pressure to drink alcohol to a dangerous level. You can now sign up to potentially get killed in an instant at 18, but you can't intentionally give yourself cancer slowly. Kids have better reflexes than seniors, but are also more reckless (imo both ends of the age spectrum should require more frequent driver's testing and restrictions).

So since it's all arbitrary, either we make everything one age, and 18 is a common median of the age-restrictions, or we ditch the restrictions entirely and rely on more extensive and expensive regulations based on individual development.

I, personally, am itching for progress. In my lifetime. What history has proven is that progress is never achieved without bloodshed.

Though there is one very easy step the US at least can take that isn't bloodshed: STOP SELLING WEAPONS TO ISRAEL.