According to www.handgunlaw.us, Idaho law says you have to be 18 to possess a handgun.
I am not an expert on Idaho law, but I see no further restrictions.
You have to wait until 21 (like most states) before applying for a concealed weapon permit, or to purchase a handgun from a licensed dealer. That is a restriction on the dealer, not you.
I'm sure an Idaho expert will be along shortly.