I'm in a minority here. People obviously vary and one fifteen-year-old could be sensible and mature whilst another... well, not. The older boy could have left the 'daft teenager' phase but... perhaps not.
I have knowledge of a friend's daughters who, when left alone for a week, had a party which rapidly became out of control and a beautiful house was trashed (to the extent that a marble fireplace was forced off a wall and paint split over carpets).
There are also the excesses which these boys, now free of parental supervision, may feel free to indulge in. Again, perhaps not, as it depends on their personalities/friends/upbringing - but who can tell which it will be?
Personally I wouldn't be happy about leaving boys of this age alone and yes, I understand that one can vote/join the army etc., etc. I would say it's a bit of a lottery as to what could happen and overall still think that peer-pressure could encourage uncharacteristic behaviour that could damage their home and themselves!