Grandchildren becoming independent

It depends on the individual child IMO. You cannot generalise.
We sent ours on long train and plane journeys from an early age (8,9,10) but didn't leave them alone in the house until they were quite a bit older.

Friends' stories of houses wrecked didn't help!
I begged my DM to allow me to stay home when she went on holiday when I was about 15 but after a couple of hours I was lonely. Luckily an older sister came home to keep me company.

I think that's quite sad, as basically the parents are saying they do not trust him - although he is old enough to vote, marry, earn his own money etc etc.

But it does, of course, depend on his previous behaviour and maturity. If mum is a worrier then it would probably ruin her holiday to leave him at home.

We left DD at home when she was 16. She was allowed to ask three friends round providing we approved them in advance. She was responsible and everything went as planned.

The time she messed it up she was 18 and had passed her driving test - and no, it did not involve alcohol or sex.

It has much to do with maturity. I rode a London bus on my own aged 8 years. I went on holiday with my friend aged 15 years, no adults with us. Children today are mollycoddled (no criticism here, we all do it) so they are not able to cope on their own until much older. It's up to the adults to judge how mature they are. I'd say just make sure they've had a fire drill, don't answer the door, know what to do in an emergency and can get an adult there quickly if required.

My granddaughter 16 had a weekend alone this last month I was just less than 1/2 mile away and could be there in 10 minutes I offered to pop in and out but that wasn’t wanted I thought she might not like the night time alone but she was fine
I always try to put it into perspective because my grandad altered his birth certificate and was fighting in France in WW1 at 16 and three quarters riding a motorbike as a messenger between the officers and front line apparently he was very lucky as I ve been told that most of these young out riders didn’t come home

I left my almost 16 year old for a weekend when he refused to come with us. He was sensible and there were no parties. He would have stayed home playing video games and watching films.
When he was 18 I left him alone with his 15 year old brother.
They knew the neighbours well if they had a problem and they were fine.
I don't understand why your GS can't be left if there is another adult in the house.

One of our DD’s didn’t want to go on a certain holiday with all of us ( she was 16) so she was allowed to stay at home, with two phone numbers (of old friends of ours that she trusted) in case anything went wrong.It’s not the age as such, but how mature they are.

We left our nearly 17 year old daughter on her own when we went to Cornwall for a week back in 1991. Her 15 year old brother came with us. I think a girlfriend stayed with her, and my mum lived a few minutes walk away. She had left school and was starting college in the September. She wasn’t a goody goody, but I felt she was responsible and mature enough. She didn’t let us down.
Our son however, a few years later! The lounge carpet seemed to have been shampooed before our return? and our lovely neighbours said they had been a bit concerned as windows had been left open downstairs for long periods when the house was empty???.

I think it's plenty old enough.
He'll probably not even leave his room.

We left our sons on their own for the first time when they were 17 & 14 but my parents were only a few doors away so didn't worry. We then left the youngest on his own a couple of years later after his brother had left home and although he'd been ok he'd had a party and there was a broken back door and an ink stain on my white sofa! I think it depends on the maturity of your GS and whether there are others around who can keep an eye on him.