Depends on the 18 yrs old, I know some that I wouldn't. But I know some 16 yr olds that I would trust ( if it were legal).
My daughter when she turns 18 will be getting her license to carry and will be getting to go thru safe and pick what gun she wants and if I don't have one she likes I will make sure she gets what she wants.
My daughter just turned 18, I bought her a Glock. She doesn't want to carry it yet as she isn't ready. I told her that was fine and I agreed if she didn't feel ready she didn't have to
carry it. She will get her permit and take some training and when she decides she is ready she can carry it. I agree with others that have said it depends on the 18 year old, I carried when I was 18 and didn't do or say anything stupid but had friends that should not have been allowed to even look at a gun.