Probably because they don't use contraceptives, or any other measure to try to limit late child-bearing. Nor do they use pre-natal testing to identify and abort Downs syndrome fetuses. If older women have babies (the natural way) there will be a high incidence of Downs syndrome -- same for Amish and non-Amish women.
Let's face it, there is also a certain level of inbreeding. Although, I have heard that they are trying to get some new blood by way of "swapping" with other communities across the state and Ohio. I am not sure if this is true or not though.