when HP was in full bore (pre book 7) Borders' suggested reading list said HP should be 11-12th grade.
my boss thought I was nuts when I said 5-6th grade then redid the entire district's list.
I'm pretty sure the books were designed so the "sweet spot" age for reading each one would be to be the same age as the main character, who conveniently has his birthday at the start of every book in the series.
Not that they can't be read before or after, but this seems like the intent.
Ho no they were not. Absolutely no design. She's wrote the first one, got lucky and then wrote more. There's absolutely no tangible story arc frombthe first to the last book. Bloody lazy mess and horcrux my arse!
I agree with no story arc between books, but she did write with intent towards an audience who would age with the character.
I'm not really a fan of Rowling (I am trans, although there are other reasons) but there are a handful of things she did at least competently, if not well.
It was in the publicity of the very early books, it absolutely was the plan fairly early
Before first publication? Probably not but it was part of the plan for the sequels
She might now be a vile person but she and her agent did have a plan