I suppose it depends on what you mean by looking after. I don't go away and leave them alone for weeks on end, I don't expect the elder to be responsible for the younger's school lunches or dentists appointments.
But I do let them go into town together and have done since the elder was 12 and the younger 9. And I have been leaving them alone together in the house for an hour or so since approximately the same age, perhaps a year or two earlier. Dh left the now 14yo and 11yo for several hours late at night last week when he picked me up from the airport- their choice, they could have come. Of course the 14yo was responsible in the sense that she would have rung emergency services if he had been taken seriously ill- but then he would have done the same for her.
I don't really see it as the elder having to do a job: a 9yo is not a baby and doesn't need much looking after. They get on and like each other's company and usually get a choice about whether they want to go with me/stay at home with me or not.
I have let them play outside in the summer without immediate adult supervision (small, very safe island) since they were, I suppose, about 7 and 4 respectively.
Don't have any younger children but would have no qualms about letting 14yo babysit a toddler if I had one to hand, she'd probably enjoy it.
I grew up with several siblings, including two younger ones, and we spent a lot of time tootling round after each other: I don't remember it as a massive chore. I suppose my parents were reasonable in what they expected and a small measure of responsibility came with more freedom, so I was happy with that. In retrospect, I don't think they asked for more than was fair and sensible.