The point of age restrictions is safety, and the reason we don’t make insanely high age restrictions is because we balance utility against it.
Whatever the proper balance is for a particular endeavor, it shouldn’t be affected by the fact that a different endeavor has a different age at which the balance is struck.
If you’re so concerned about the lomdus of being m’chalek between a child and an adult, let the age of adulthood be 21, but we’ll let 17 year old kids drive, and 18 year old kids go to the army, but don’t compromise on safety or utility for some relatively arbitrary number (arbitrary because it’s not the best number for a particular law, but chosen because it’s appropriate for a different one).