What are the carrying laws for knives/firearms in the 18-21 age range?

Not enough facts to answer. You don't even identify what state you are living in. Every state has its own laws.

Google the statute in your state for possession of a weapon. In New York there is a long list of prohibited weapons including a dagger, gravity knife, switchblade (as opposed to a legal folding knife it has an assist to open it with gravity and a shake or a button) or a Chinese star. It is not dependent on age but rather the nature of the knife or weapon. There are knives that are legal to sell but not carry.