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.
How long should a mother breastfeed? The American Academy of Pediatrics (AAP) recommends that breastfeeding continue for at least 12 months, and thereafter for as long as mother and baby desire. The World Health Organization recommends continued breastfeeding up to 2 years of age or beyond.