As I mentioned before, we've been reading the book "Love and Respect." I keep wondering, though.... How do you show respect for a young adult who sometimes does, well, dumb things. I've decided that I need to have a boundary for my 18 year old men. My husband and I try to only intervene if we believe the boys are doing something that is "life-threateningly" dumb, that is, doing something that could negatively alter the course of their life. If they are just doing something that isn't up to our "expected standards of responsibility" (i.e., dumb) then perhaps it's just a demonstration of independence, or a personal preference that we wouldn't make ourselves.
When our men turned 18, we want them to make choices for themselves based on the instructions they have received over their lifetime. We try to show respect for the men in our household and only intervene when we believe it is necessary for their future well-being.