One son stayed by himself at night when he was 18, and one started staying the nights by himself at 17. Before I met/married DH, there was no reason for me to leave them alone at night. Now my youngest is 19, but I still ask someone I work with to be his emergency point of contact, in case a medical emergency comes up. I also give them a key to my house, because my kids are notorious for losing house keys!
A funny story, the last time me & DH went to Food & Wine, my son was by himself (he had to work). Well, he asked if he could have one of my husband's ASPs (Police Baton) to keep at his bedside "in case of an intruder". Our house is alarmed, and my son is 6'3 1/2 inches, and about 250lbs....just seeing him is scary sometimes! Anyway, my husband has two ASPs...one of them has a Pepper Spray attachment on the bottom. Well, unknown to me, DH gives him the one with pepper spray.
So we are at EPCOT, and I get this text: "I just pepper-sprayed myself".

After he showered and ran water in his eyes, I asked him how in the world he did that, and he told me he was showing his friend the ASP and was trying to close it, and pressed the button. Oh my gosh, I was so
livid at DH for giving
that one to DS, without saying anything!! I can laugh about it now....but definitely
not funny at the time!!
So far, that's the worse thing that's happened when we've left him alone.
I worry about our cruise this Fall. He'll be going to a local college, and my biggest fear is he'll get in a car accident while we are out to sea and can't get to him. I know he'll be 20, but you NEVER stop worrying!!! I'll do the usual, ask someone at work to "be there just in case", and make my son put his name and phone number under ICE #1 (In Case of Emergency, 1st Contact), in case something happens to him.
As a note, everyone should have ICE #1, ICE #2, and ICE #3 in their phone. That way if they are found unconscious from an accident or something, the police can look at their cell phone for ICE# 1, so they know who to call.