Some vets will neuter male bunnies as soon as their testicles drop. My bunny’s showed up at 12 weeks, and if I remember right my vet neutered him a couple of weeks later.
Like Bumblebunny said, I also remember vaguely a post about member whose bunny had babies very early. Not sure if the age though was correct in the first place or if there was some question about that. But obviously if you have a male and female you have to be extremely careful.
You won’t able to get them fixed before they will be able to have babies though because even if you did have your male fixed early, he still has sperm that is viable for up to month and he can impregnate her even after he’s been fixed, and you won’t be able to get her spayed by then most likely. So unfortunately, you will have to separate them soon.
This article from he house rabbit society http://www.rabbit.org/care/babies.html says that it is possible for some females, though not as common, to become pregnant as early as 10 weeks. (though 12 weeks is said to be the earliest by other experts). But really, not sure if I would want to to test out first hand which experts are right. 