It is legal for those ages 18-20 to openly carry a firearm in Nevada. No law prohibits it. In Nevada, when it is not illegal to do something, it is legal to do that thing. "A crime is an act or omission forbidden by law and punishable upon conviction..." NRS 193.120 Don’t believe me? Read the NRS for yourself.
Confusion about this stems most likely from concealed carry being restricted in general to those over 21 (more this later) and the fact federally licensed gun dealers (FFLs) can’t sell handguns unless you are at least 21. It doesn’t help that California and other states have raised the firearm purchase age to 21 as well after the Parkland shooting.
The 1968 Gun Control Act made it illegal for FFLs to sell to 18-20 year olds ; see 18 USC § 922(b)(1). In Nevada, until Jan. 2, 2020, it was legal for those under 21 to buy handguns legally in private sales. However, with the enactment of universal background checks that banned private sales, all guns sales must go through a dealer for a background check. Since dealers can’t process a sale/transfer to someone under 21, the perfect storm of denying access to 18-20 year olds to handgun purchases was created. Someone in that age bracket can get around this by having an exempted close relative gift the handgun to them.
Now for concealed carry, one must be 21 years or older to have a concealed firearm permit issued to them. NRS 202.3657. You generally are allowed to apply prior to your 21st birthday, such as taking the class <4 months before your birthday and submitting your packet so that permit is approved a short time after you turn 21. Contact your sheriff for details.
For those 18-20 who are in the military, or the rare honorably discharged veterans at that age, they can also apply for a concealed firearm permit (NRS 202.3657). This was changed in 2017. If you’re not in the military (active, reserve, or Guard), got a “big chicken dinner” bad-conduct discharge, other than honorable discharge, forget about it. The law requires “under honorable conditions,” so that leaves an honorable discharge or a general discharge under honorable conditions.
Note that these 18-20 permits are not honored by most states and a lot of the constitutional carry states require people to be over 21 to carry concealed. Nevada does honor Utah's 18-20 permit and go over to FrontierCarry,org to see if Utah does likewise.
So once again, it is perfectly legal for someone 18-20 to openly carry. Police are generally educated on the issue and won’t harass you. If they do, politely explain to them or if they refuse to listen, respectfully demand a supervisor. Should worse come to worse, and it probably won’t unless you are pulling some sort of stunt or are major asshole, you file a complaint against the officer when it’s all over and call a civil rights attorney. Don’t worry about the cops busting your chops; open carry has become a non-event in Nevada.