I've got several items on a trust and was curious about the legality of benefitiaries having access to them.
Can they have access to the items?
I know trustees can, but I did not add any of my benefitiaries as they are not 21 yet, though they are over 18.
how to I add trustees/benefitiaries?
Can a benefitiary also be a trustee?
And in the event that I am incapacitated, who would get my nfa items?

It's possible for some beneficiaries to also be co-trustees, but there can't be complete overlap.  IOW, if all of your beneficiaries are also trustees, the trust is probably invalid (depending on your state's laws).  Beneficiaries don't have any right of access to trust property, though, until whatever event happens that gives them that right (which is usually your death).

As to your incapacity, it depends on what your trust says.  Most likely, your co-trustees would have access to your stuff (the same as if you weren't incapacitated), but the beneficiaries wouldn't get anything until your death.

Another question I have is: if the person is not 21, can they be a trustee? They are over the age of 18 which you can technically own the stuff, just not transfer from a dealer. My brother and girlfriend are both 20 and I'm just trying to keep myself out of hot water. Changed my safe combo just to make sure, although it sounds rediculous.

I would absolutely not make your girlfriend a trustee.

That really is a question of state law.  Given how much LA law differs from the rest of the country, I wouldn't care to guess.  I agree with the others advising against adding your girlfriend as a trustee.

Guess I should rephrase. We are engaged. But I see the problem with it regardless.

It purely depends how your trust is written up. My lawyer wrote mine so I'm the sole trustee but there is a list of "present beneficiaries" that have full access to use trust assets even while I'm alive.