For Denver, see below.
The minimum age for open carry in Colorado is 18 and does not require a permit. Open carry is more restricted than concealed carry is (this is a reverse from most states in the West). Local governments can ban open carry at government buildings and public property (such as parks). Signs must be posted at each public entrance.
29-11.7-104 Regulation-[open] carrying-posting
A local government may enact an ordinance, regulation, or other law that prohibits the open carrying of a firearm in a building or specific area within the local government's jurisdiction. If a local government enacts an ordinance, regulation, or other law that prohibits the open carrying of a firearm in a building or specific area, the local government shall post signs at the public entrances to the building or specific area informing persons that the open carrying of firearms is prohibited in the building or specific area.
Many Colorado municipalities ban open carry to the fullest extent of the law. While areas such as public streets and sidewalks generally cannot be designated 'no open carry' zones, "specific area" is broadly interpreted to include parks, trails, and public plazas. Local governments are prohibited from regulating concealed carry only.
'No guns' signs do not have the force of law on private property. If you openly carry past a 'no guns' sign on private property, you may be asked to leave or disarm. Failure to comply would only be trespassing. Signs on private property may cite 18-4-504, which is simply trespassing.
Menacing (18-3-206) is if, by any threat or physical action, a person knowingly places or attempts to place another person in fear of imminent serious bodily injury by the use of a deadly weapon (felony). Case law indicates this section is meant to be applied when someone uses a firearm to essentially threaten death in aggravated circumstances (assault, robbery, road-rage, etc.).
It appears that under very specific, narrow circumstances that one could be charged with a crime for open carry, but no reports suggest that this is a problem in Colorado and furthermore, case law indicates that exacerbating circumstances would likely be required to elevate open carry to a different crime. An intentional act, such as making threats or drawing a firearm not in justifiable self-defense, would be required on behalf of the carrier, not simply a fearful reaction from a hoplophobia third party.
Colorado's state preemption of local firearm laws is rather weak. As you have read above, Denver's exemptions border on ludicrous. Additionally, unlike other states, which usually reserves firearm regulation power to the legislature and grants limited authority to local governments, Colorado only prohibits local governments from making certain laws in a very narrow field. Local governments are prohibited from regulating concealed carry only. 29-11.7-101
See also 29-11.7-104 regarding open carry lack-of-preemption protection.
Open carry is illegal in the City and County of Denver 38-117(a) DMC
Denver has banned open carry since 1973, under the premise that Denver is a 'special' home-rule city and somehow different than the rest of the state. Arguments seemed to be based on the fallacy that an openly carried handgun equates target shooting and that allowing open carry in the city would lead to people getting shot. The Supreme Court found that Denver has the right to regulate open carry because it is urbanized. In other words, Denver residents are less entitled to full Second Amendment rights than other Colorado residents.
The Supreme Court's opinion allowing Denver's strict gun control regime is based on the fact it is organized differently than most cities. Under home rule, it is not subject to certain limitations that other cities are subject to. While that issue is one unique to Colorado, the arguments to support the validity of the anti-gun ordinances are specious and are a stunning abuse of logic.
The City and County of Denver is a home rule city created and organized under Article XX, Section 6 of the Colorado Constitution (the “home rule amendment”). Under the home rule amendment, a home rule municipality has the supreme power to legislate in matters of local concern. Historically, Denver has had a range of ordinances controlling various aspects of the possession, use and sale of firearms in the city. (CO Denver Ruling)
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