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.
Mother cats begin to gradually wean kittens three weeks to a month after birth. The kittens begin to eat solid food and are usually fully weaned after six to eight weeks. Kittens also begin to develop their hunting abilities at this age; feral mothers begin bringing live prey to their kittens around this time.