Originally Posted by SavannahKitten

Unless a vet is specialised (eg. cardiologist), then take his advice as you would a GP. The vets can advise on this fact, that fact, and the other fact - but they're not trained in behaviour - that takes an observer (the breeder).

My experience has been varied. I stick to the 13 week rule (though this is sometimes by shortened by a couple of days to help the new owner out if the 13 weeks falls just after a weekend).
I'm sat here with seven Bengal kittens on my lap. Three of them are nine weeks old and there's no way I would rehome them. Bengals are mucky buggers. These guys are covered in food and poo (and yes, they are sitting on me!) and look a state. They haven't learnt to clean themselves properly yet and their mucky habits can lead to bouts of the runs.

We also have two eight week old Abyssinians. Their cleaning and litter box behaviour is impeccable. They don't seem to want a warm lap to sit on -they're too busy 'killing' everything in sight. Disregarding the injections side of things, these two would be able to go to a new home with no problems at all.

When it comes to leaving mum I also have different views than other posters.
Our kittens leave their mums when they want to - or when mum wants to leave them.
We have a mother who gives birth to wonderful show quality kittens, but is trying to get rid of them after five weeks. Six weeks is tops. If she is shut in a pen with them she howls to get out and turns her back on them. Then she will pace around the pen trilling at them to try to get them out. I see this as her trying to 'force' them to be independant. She has had two litters and has done it with both. She will get into the pen filled with milk but will refuse to feed her kittens. I can't say her kittens are bothered at this point though - as they're usually early weaners.
Keeping a mum with their kittens for the full 13 weeks just isn't done here - I haven't got a mum that is that maternal - or kittens that are that needy. Once the kittens are weaned they are looking to us for warmth and cuddles.

Finally - we did have a woman come round to look at our Abyssinians - twice. Second time she brought her husband with her, who whipped out his stephoscope and started inspecting the kittens. My husband was most surprised. It turned out the husband was a vet, but they didn't ask permission to do it. This same woman had asked that she take our Abys home at the age of five weeks, stating that she was a qualified veterinarian nurse which meant they would be in 'safe hands'.

The arrogance of these people can be amazing, so once again I would say when it comes to rehoming, take the vet's advice with a pinch of salt.