Hi Mazzyruss - like others, I think it does depend on the breed. Siamese and orientals do tend to be rather precocious.
IMHO, however, as Anele correctly says, if you get the male neutered as soon as you can arrange it, that would help avoid potential problems.
There are vets who will do this even if your nearest one disagrees. The male operation is a more straightforward one for a vet to perform and is not so risky because it is quicker to complete. The female can be spayed at the right time, whenever you consider it right/the vet advises.
Please let us know how you get on ... good wishes
