That's an interesting question! These are just thoughts that come to mind when I hear you ask this. My credentials? I've raised several kids, all of whom are independent and don't write home for money (one measure of success I suppose, lol). They all seem to be making reasonable decisions at the helms of their own lives, so I would guess that would be where you want your 18 year old to be. There are all sorts of factors that I don't know, so ignore this if it doesn't fit. Keep in mind that much of what an older person will tell you is based on how many times they did it wrong and figured out that what they had done didn't work. Some of it is based from having been that young person and having an opinion about interactions that were helpful or not. From that reservoir of experience I speak. The suggestions I offer might also be wrong, but here goes. See if this helps any.
If I were looking at my 18 year old saying that, I think I'd explore what he/she is feeling and offer empathetic responses. If that other family is a good influence overall, I'd encourage bonding with them as much as they are able. What do you have to lose there? My kids had significant families they felt safe with. In a multitude of counselors there is safety, right? If the family this 18 year olds wants to join isn't a good influence, I'd explore with him/her what possible consequences could come by being full time in that environment, things like would it really help him or her in light of long term goals. Granted, at 18, I myself had NO long-term goals, but imagining that I'm the 18 year old, I do think someone actually listening to what I was thinking/feeling, someone who could guide me with very gentle questions, even to the point of asking permission to ask and making frequent assurances that nothing I was going to say was going to be used to stab me, might have made for some good. It would have likely bonded me closer to that someone (you in this case) who cared enough to take time to explore who I am as an individual and who would trust me to keep the reins of control of my life in my own hand by staying away from the parental control button in support my autonomy. At age 18 a person is considered old enough to join the military, so it's beneficial to make sure that the shift from parental control to self control is both in place and respected. Being along-side instead of confrontational may also give me, if I'm that 18 year old, reason to start thinking about the reality of a future and how I would want to handle it.
