Here are some of the biological reasons that make kittens emotionally predisposed to accepting humans so readily at this age:
During the weaning period from 4-8 weeks, kittens start the transition from complete dependence on mom to being completely independent. Some have likened the impact of this stage to the transformation of a Caterpillar to a Butterfly. Nature converges many things to make the readiness to quickly learn a completely new way of life from mom very strong at this point. For one thing, at this age, mother's milk is no longer enough nutrition to support the kitten's rapid growth. The kittens are anxious to accept a new source of food energy that can meet the demands of their growing bodies. Using food to tame kittens (as demonstrated in the Urban Cat League video TOUGH LOVE) is very effective at this age when they are craving nutrition and mentally open to new things. Taming to humans obviously wasn't part of nature's plan for this period in a kitten's life, but this is when that is most successfully accomplished.
It's during this weaning period that Mom would normally start bringing prey to the kittens to introduce a new energy source and start to teach them to hunt for themselves. Nature encourages this process by making mom increasingly intolerant of nursing, thereby increasing hunger in the kittens, adding an eagerness to learn to hunt. While nursing, moms have been steadily losing weight, using up their body stores. Nature needs her to stop nursing and caring for the kittens to regain her condition to be ready for the next breeding cycle. The growing teeth of the kittens, and their claws kneading her belly add a lot to make mom more interested in teaching them to hunt than continue nursing. Along with hormonal changes in mom, all of these influences come together for a very efficient transition to the kitten's independence. If you want to tame kittens for indoor life and/or adoption, this age at 4-8 weeks is your crucial window of opportunity.
To further impress the importance of this timing, science has shown that while kittens are very ready to learn and change habits at this age, they are also very stubborn to later change or give up any impressions formed at this age. It is important to create the good association with humans before they have learned otherwise. Once a fear of humans is ingrained, it is very hard to change their young minds after the fact.
It is also around 6 weeks when the kittens start leaving the den, that a new communication with mom develops as part of the learning process. Mom growls when there is danger and the litter scatters and hides until mom signals that the danger has passed. If humans are a perceived danger for mom, the kittens will be learning that directly from her. They can re-learn differently with the prompt introduction of a counter incentive of food and nurturing from humans, but they aren't easily convinced to change their minds after this second month of life has passed.
The period between 4-8 weeks is when nature has biologically predisposed kittens to learn and absorb basic life lessons at warp speed. Whichever teacher gets there first gets to have the strongest influence on their opinions for a lifetime.
Cats are very adaptable and will change their opinions over time but nature has stacked he deck against any "Johnny come lately" lessons in the joys of a life with humans.
Start the taming early, be consistent and get it done as young as possible!!! Otherwise be prepared to be very patient and forget any time line. It can be done, but there are no guarantees as to how long it might take to gain an older cat's trust.