It worries me that anyone thinks a kitten older than 8 weeks is too old. Don't they realise that they grow up anyway?
Mosi was 14 weeks old when I got him and he was still a tiny, playful little thing. I don't feel I missed out on his kittenhood. Jaffa was 8 weeks old but I think that was because he was in a shelter and wasn't with his mum so he was probably better off in a new home than in a pen there. I think 10 weeks or older is best for rehoming. I can understand rescues needing to free up space so letting them go at 8 weeks. I don't think they should go any younger though, unless they are orphaned so not with mum anyway (and so long as they are rehomed with a sibling). Over here, GCCF guidelines (which breeders agree to when they register their kittens) say that kittens should not be rehomed until they are 13 weeks old. That is partly so that they are fully vaccinated by the time they go to their new homes.