Federal law states that You must be 21 to purchase a handgun:

http://www.nraila.org/GunLaws/FederalGunLaws.aspx?ID=60

This was a central provision of the Gun Control Act of 1968. So, you can't really complain about your state being the problem, the problem is in Washington. This is pretty widely accepted though, so I'd be highly surprised to see the age lowered.

Of course, states like SD can dictate their own carry policies. Were I to sell a handgun privately, I'd think I'd want to see proof of age 21 first, even if I lived in MI where it's reported above that an 18-year-old can purchase from a private individual.