Because nature says women shouldn't have kids at that age. It's abnormal and unhealthy for the woman and child (who will be lucky to be born without health problems).
It's pretty disgusting on a moral and social level too. A sad only single granny (not literally, hurr) bringing up a kid... these sad people missed the boat, and should accept it.
Also, adoption is banned for over 45s or something, so this should be similar.
It shouldn't, but parents should be charged with causing grievous bodily damage if they insist on going through with a pregnancy when there is a significant risk of the fetus becoming infected with down's syndrome and the child has down's syndrome.