ETFs tracking the Barclays Capital U.S. Aggregate Bond Index are presented in the following table.
The real reason kittens nurse only for 6-8 weeks is because of human intervention. Since they are capable of eating solid food, people go ahead and remove them from their mothers to place them in homes. But 6 weeks is still a bit young - even the Humane Society won't place kittens until at least 8 weeks old.
By 8 weeks of age, the kittens are eating solid food. Older kittens who are still with their mothers may occasionally nurse, but this is more for comfort and reassurance than for nutrition. In general, kittens should not be removed from their mothers until they are at least 8 weeks of age.