16 and 17 year olds have even less of a risk assessment capacity than 18 year olds (worth noting that this faculty fully matures by around 25), but jobs where bad decisions have more impact aren't the place for people new to the workforce to cut their teeth; there's too much risk not just to themselves (which we're disregarding here; people should be fine making whatever stupid decision they want) but to the people around them.

Start them off doing jobs that have responsibility but less consequence. Then progressively ramp up the responsibility and accountability as they grasp it.

That's your rational argument.