When did lupin become a werewolf?

It's impossible to say precisely when Remus Lupin became a werewolf. However, the information we have from the Harry Potter books suggests it happened at a very young age.

Here's what we know:

* He was bitten as a child: J.K. Rowling has confirmed that Lupin was bitten by a werewolf before his eleventh birthday.

* He remembers being bitten: Lupin doesn't remember the actual biting incident, but he remembers being in the woods, seeing Fenrir Greyback, and being afraid.

* He was already a werewolf at Hogwarts: Lupin attended Hogwarts at the same time as Sirius Black, James Potter, and Peter Pettigrew, who were also his best friends. This suggests that he had already been bitten before starting Hogwarts.

Therefore, we can safely assume that Remus Lupin became a werewolf before he turned eleven years old.