alt Hacker NewsI'm of an age to have grown up like that, and one of the real drags was having to take my 3-5 year-old sibling along on whatever activity the rest of us were up to. ("But mom, we're building a fort - do I have to?") The other kids with younger siblings did the same. As I recall, at five I wasn't to leave the yard by myself, but as a mixed-age group of ~3-11 year-olds, yeah: we ran around all over the place together. The older kids took on responsibility for the younger ones.
So, yeah, that's eighties suburbia, and my sister wasn't less than two. On the other hand, if there'd been a larger age difference (and, maybe if I'd been a girl? My mum was more progressive about gender roles than most of her contemporaries, but still) I expect she'd have been entrusted to me earlier. Starting at the age of ~4 I'd been left alone with my sister for up to an hour while she napped, with the instruction to run next door to get my mother if she woke up.
By the way, I think all of that was fine.
It was not like that in the London suburb I grew up in in the 80s. I think it was more common in smaller and poorer towns - maybe because people had less space at home. I agree the age gap probably affects it a lot. Keeping an eye on a younger child at home with help near by is very different from going out.
Kids spending a lot of time in front of the TV was already a concern in the 80s so screen time was definitely a problem back then. IIRC the average amount of time children spent watching TV a day was about five hours a day in the UK.