I'm not sure. I seem to recall that the language implied that Dumbledore co-created it with Flamel, which would mean 20th century, after all; it's not clear how long wizards naturally live, speculation based on apparent size of Magical Britain to the side.
But it's also possible Flamel created in the 1600s, this is how he survived to the 1900s, and the brilliant young Dumbledore hearing of Flamel's stone, independently reinvented it. Or something.
I'm not sure. I seem to recall that the language implied that Dumbledore co-created it with Flamel, which would mean 20th century,
The wording in book one which you may be thinking of just says that Dumbledore and Flamel did alchemical work together. Flamel's construction of the philosopher's stone is not mentioned until much later when Hermione finds the reference and there it just says that Flamel is the only person known to have made one.
- This thread has run its course. You will find newer threads in the discussion section.
Another discussion thread - the fourth - has reached the (arbitrary?) 500 comments threshold, so it's time for a new thread for Eliezer Yudkowsky's widely-praised Harry Potter fanfic.
Most of the paratext and fan-made resources are listed on Mr. LessWrong's author page. There is also AdeleneDawner's collection of most of the previously-published Author's Notes.
Older threads: one, two, three, four. By tag.
Newer threads are in the Discussion section, starting from Part 6.
Spoiler policy as suggested by Unnamed and approved by Eliezer, me, and at least three other upmodders:
It would also be quite sensible and welcome to continue the practice of declaring at the top of your post which chapters you are about to discuss, especially for newly-published ones, so that people who haven't yet seen them can stop reading in time.