The problem is there are probably adults who shouldnt be left alone and unattended!!! or others that would not possibly stay overnight in thier own home on thier own! so obviously teenagers are the same.
Once you get to a certain age its more about the individual than how many times they have orbited the sun.
So the questions to ask are:
Would I leave them during the day? Houses are actually more likely to be burgled during the day while everyone is out at work! houses also burn down during the day. Night time isnt massively different in terms of risk.
Can I trust them to lock up the house?
Are they able to cook (not necessarly a full roast dinner) but either chuck somethinig in the oven, boil some pasta, reheat something in the microwave. Can they use a sharp knife to cut salad up etc?
Would they throw a party? would they have friends who might turn up and throw a party?
Do they know what to do in an emergency eg if the power goes out? is there a torch handy? Do they know how to get out of the house.
How soon could an adult - friend, family or neighbour get to them in an emergency eg illness or accident?
Do you have a contact process in place? we have a policy that we will text every so often, if they dont reply to the text withiin 10 minutes then we ring and then if we were to get no answer we would send someone round.
Teenagers should be being equipped with skills to help them trhough life. I know someone who broke thier leg and none of the age 18+ men in her house could even chuck a ready made lasagen in the oven or make a cup of tea! so many kids are going to uni now and just have no idea how to do anything not even boil an egg or wash thier clothes. That is a failing on the parents (special needs aside).
My 12 year old went to scout camp last weekend, they had to put up thier own tents, cook thier own dinner with food they had planned and taken along themselves, amuse themselves, and cook thier own breakfast. Obviously adults were there overseeing but the point was that they were doing it all themselves and an adult would only step in if something dangerous. The kids all did fine, some didn't cook thier pasta long enough or didnt wear enough warm layers at night but that is a learning curve for them and next time they will do it differently.
Parents needs to be gradually teaching thier kids skills so that when they do need or want to leave them alone for a day or overnight they can be more confident that thier children are equipped to cope.