I'd check with your state laws on this. I've been told by an insurance company's attorney that if an 18 year old adult has the title to the vehicle, they are the ones responsible in an accident.
If you're worried about losing control of the vehicle, put your name down on the title as a secured party, just like a bank. This way they can't sell it without you signing off of it, and you wouldn't have the liability expusure that an owner would have.