How old do girl bunnies have to be to have babies?

Well I was told told 6 months but i know that's not true as my female rabbit gave birth unexpectedly at just over 4 months old so she must have got pregnant at just over 3 months old, so i would definitely keep males and females apart from each other before they reach 3 months in future, as its not recommended for them to have babies this young.
A little time that they have access to a boy bunny.