At What Age Do Kittens Become Less Adoptable?

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I've noticed that in helping my friends find homes for kittens, that everyone wanted them when they were 8 weeks old. But there were steady adopters until they were about 16 weeks old. Yet my friends were super picky and so only 2 of 4 kittens have been adopted out. Now that there are 2 left, they are 4 months old and there isn't much interest in them. They still look pretty much like kittens to me, so I'm wondering what the difference is. If you're going for the kittens imprinting on you more, that critical point is known to be at 7 weeks, so after 8 weeks I'm not sure why the age seems to play such a big factor. But I guess people want them to stay kittens for longer.