I liked the way you phrased that--in a perfect world. I've listened to two different ways of thinking about it.
Personally, I think eight weeks is acceptable, although I still think they are better adjusted if they stay until 10 or 12 weeks. Just make sure he or she is spayed or neutered before he leaves.
Some people say because they are bottle babies and with no mother to teach them cat things, you can go ahead and adopt them out at 5 or 6 weeks just like we did in the old days. I don't really agree with this. You ARE momma; you and your cats continue to teach him things he needs to know until he leaves. I believe older is better for him, but not necessarily for you.
I have one kitten that will turn 18 next month. I think she's almost old enough to place for adoption. I will say, the sooner you can find a home for the kitten, the easier it will be to let him go. I used to have an arrangement with other foster homes. I cared for them until they were about five to six weeks and they cared for them after that. For me it is much better. Although I love them all, they don't start really developing much personality until about five weeks.