Legal without Permit requirements except in the City and County of Denver. Also there are places that are marked off limits to open carry. Carrying a gun in a private vehicle is legal. This does not extent to public transportation. No permit is required. The minimum age for open carry is 18.
Open carry is the ability to carry a gun in plain view. Some states require a permit to open carry, and some do not, but most have restrictions on where it is legal to open carry.