Yeah, they can breed as young as a year and less than 100g, but fertility is often reduced (though not always). I aim for 150g+ for males, usually 200g+.

And as you found out, corns are opportunistic breeders!! I've found that often just the natural changes in the temperature during the winter can and will easily trigger them to want to breed, even if you didn't brumate them. Good luck with that pair!