I think the lack of standards in situations like this cause the most frustration. If 18 is an adult then 18 is an adult. If it's 21 so be it.
I also think exemptions are ridiculous. The law should apply to everyone uniformly.
I knew a person that went through the police academy and was employed at 19. The had to have their parent purchase ammunition so they could train with their department issued duty weapon because in my state you need to be 21 to purchase handgun ammunition. Are they an adult or not? But if they aren't adult enough to purchase ammunition then they aren't adult enough to be a cop.
I don't have all the answers but the laws should apply equally to all.