at what age should male and female rabbits be separated to keep a pregnancy from happening. thanks.

Immediately.

I don't actually know from experience but my husbands' family had two rabbits and nearly immediately had fifteen or so. And after ever birth, within days it seemed, she would be pregnant again. If you don't want a bunch of baby bunnies, separate them as soon as you can and get the male fixed.

If you don't, you could have 1,300 rabbits in a year and 5 million in 5 years. Looks like Elexina is double right (though , strictly,maturity is reached at 3 months or a bit later , depending on the breed) See:

Bunny births etc