The adopter wrote back to say that the cat had been sterilised, enclosed a receipt and a photo of the cat. For some reason it seems that something the foster said rubbed her the wrong way or perhaps there was a misunderstanding of some kind. At any rate the cat has been sterilised. She said that her vet had told her that the cats could not be sterilised till 8 months - I have to say this is not the first time I've heard that.
I did ask one vet about this once and the vet said that if the cat was indoors then they did not think it was much of a problem. However there are other things to consider - the caterwauling, the noise, and the fact that it is best to sterilise before the cat goes on heat. It only needs a little window left a bit ajar, or a cat escaping out the door to end up with a whole new litter. Obviously if the cat ISN'T on heat they're much less likely to try and make a run for it too.