In California it is highly restricted. If you search through the California section you will see how it is.
For instance, the gun must be unloaded everywhere that discharge of firearms is illegal (which is most place people are,) and handguns must be in a locked case when knowingly within 1000 feet of a school with very harsh penalties for violation. This makes it very difficult to carry a handgun in an urban environment. Government buildings are also off limits. If private businesses ask you to leave you also must comply.
This is only a very short synopsis and is far from complete.
You are right though in that you don't have to be 21.
But be sure to read all of the laws very thoroughly yourself before you do anything. I'm not qualified to give legal advice. I don't even live in California but I have UOC'ed there. Mistakes with the law are not very forgiving, especially when you are trying to get into law enforcement.