sorry it is a pistol and you would need to be 21 to purchase

Paul F here in NC a 18 year old can buy a pistol from a private party as long as the Sheriff approves his pistol purchase permit but he can not buy from a FFL and if its in a "pistol" caliber he cant buy ammo due to federal sales laws

James, I'm just gonna leave this here for ya I know Eugene's not in Idaho but still.
"Idaho statute allows residents 18 years of age, not disqualified from having a permit, to carry a concealed firearm statewide without a permit except in certain designated places including a courthouse, juvenile detention or jail facility, or public or private school unless specifically authorized."
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https://idaho.gov/laws-public-safety/gun-weapon-law/
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And Wisconsin
"Open carry is legal for any person who is 18 years or older who is not prohibited from possessing a firearm under state and federal laws. Concealed carry is legal with a Wisconsin Concealed Weapons License (CWL) or a license/permit from a state that Wisconsin honors."
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Probably a few other states too that don't enforce that but the last 2 girls I dated were from WI, and ID.
I think TN, KY, Utah, and now Texas have similar laws/policies

Eugene, you must be 21 to purchase, and yes it is legal in WA state.

Eugene, it is Federal Law by the ATF that any Handgun purchase requires the buyer to be a minimum of 21 years of age Sir. Cheers and stay safe!

Randy is correct Eugene you must be 21 years old to purchase a pistol in WA state, and the SW you asked about is a pistol because the barrel is shorter then 16", but additionally you must be at least 21 to purchase a semi auto rifle in WA so even if you found a rifle version its still be a no go. There are some great bolt action, and lever guns that you can purchase before then.
Full WA restriction here
budsgunshop.com/feeds/state_reg/washington_restrictions.pdf