Why are you breastfeeding?
I guess to me, the real question becomes why are you breastfeeding? Is it because you believe "breast is best," because it was convenient, because you liked the bond? Whatever your reason should give you your answers in stopping.
I nursed twins (with no supplements!) until they were 18 months old, am still nursing my daughter who will be 3 in july (no supplements) and my four month old son (no supplements)! While I enjoy the bond, it's a lot of hard work, but I believe "breast is best." the book "so that's what they're for" goes into great detail about formula companies and the downfalls of formula. Also, while the american academy of recommends nursing until at least one year old, the world health organization recommends nursing for at least two years!!!
Do some research and inform yourself of what you're truly deciding!