No younger than six months.
It's also recommended you monitor their size as well.
If the bunny is growing too slow, it's best to wait until you know for sure it's full size.
You want to get the females either right before or IMMEDIATELY after her first heat, and the males you want to catch before they start spraying. Sometimes though, they will go into heat/spray at a young age, but that's usually when there is another rabbt close. I had a male velvetine Rex that I never even had to fix because he never sprayed.
