By the time they were 7 they would have been given the chore of hanging out the washing and the washing line was right outside the kitchen window. Mine were all born within 4 years of one another and were, and still are, a very sociable bunch. The girls were bookworms but.if really engrossed in a book, would much rather read on the swing or on the veranda of the wendy house than in the house when the weather was good, especially if the rest of us were busy in the garden. They all liked helping in the garden even when "helping" meant dumping soil over the paths or, later, digging up the plants and leaving the weeds carefully alone! 
Maybe it would have been different if I had had an only child or bigger gaps but they wanted to be with one another and with me when they were small. In wet weather when they were indoors, I'd still find them all sitting around the kitchen table with their books if I was baking or huddled together on beanbags in the playroom if I wasn't in the kitchen.
I have said time and time again, that I realise that one can't keep children 100% safe all the time, but I think the more you leave primary age children unsupervised then the more you are increasing the odds that they could have a nasty accident and languish for too long before being discovered. It's not a risk I would take with my children, they are too precious. [shrug]
My kids have been going to music courses in the holidays for a week or more at a time since they were about 9/10 years old. They were supervised, obviously but without mummy to lean on, they had to learn to get along with other kids 24/7, to organise themselves and behave sensibly and responsibly. Not unique to music courses obviously, sports courses, cubs, brownies, pony club, whatever floats your boat.
They were at boarding school from the age of 13 and were not allowed to be "in house" that is to say, in the boarding house, without a member of staff or a sixth-former present, until they, themselves were in year 11, so 15 years old. They were allowed to go into town in free time but, until sixth form had to go with another pupil from their own year or a sixth former. (Groups of 3 minimum until year 10)
I, too, prefer a slow build up and I think that staying alone at home, even for just 30 minutes, at the age of 7 is too much too soon, that's all.
When my DC are at home and out until 2am (the older two, the youngest wouldn't be out until that time unless she was staying over with someone, prearranged, or was with her brother or sister) I know that, if they are driving, they will not drink, and if they do drink, it will be because they have arranged to stay over with someone during the course of the evening, that that person will be known to them and trusted by them and that they will phone me to let me know that their arrangements have changed so that I don't worry.
If they get into any difficulty, have money stolen or lost they know that I will turn out for them no matter the time, the weather, or whether or not I was in bed or asleep. I might add that if I am out and get caught in some mishap that either of my elder two are happy to do the same for me and I will always let them know if my plans change so they don't worry about me. 
When they are away from home I know that I can trust them to behave sensibly, so I try not to think about where they may be, it's their lives, after all.