I had several siblings so alone often seemed a bit of an unattainable dream.
Also depends on what you mean by alone: "alone for half an hour while mum went to the shops" or "have a key to let yourself in after school" or "properly alone-alone all day".
If the first, probably from around 6 (though no doubt I pleaded for it to be earlier). But then I also walked half a mile to school alone from the age of six.
If the second, I think, from age 8-9. By which age I was also playing out alone too.
The third probably from 11 or 12. By which age I was also allowed to take the boat out on my own, as my parents considered me a reliable judge of weather..
This was in the 60s/70s. Can still remember the bliss of trying the back door and finding it locked, meaning nobody else was at home. And I liked my family. I just enjoyed solitude. Still do.
My parents were neither poor nor neglectful, but they involved us in responsible tasks- cooking, DIY, fishing - from a very early age and gradually passed more of the decision-making to us so had plenty of opportunity to judge our good judgment.