Welcome to the site!
There is no law that says you can carry under 21. That's a good thing. We don't want laws telling us what we can do.
There is no law that says you can't either. That's also a good thing assuming you're over 18 and have no legal prohibitions that would prevent you from owning a gun. That means you can OC.
Your story concerns me though. What would you have done if you had a gun?
A handgun isn't a magic wand. It stops fights and does it with some finality. It won't prevent them though and killing someone is serious. Moral considerations aside, if you aren't justified in shooting someone, life as you now know it is over.
Something to think about!