I think that, as she has a mental age of 9, she is not capable of making that choice in the first place @Gingerkittykat and that's why the Court of Protection came to this sad but ultimately necessary decision 
The alternative to the abortion is that she carries a pregnancy to full term and gives birth. With a mental age of 9. How is she going to understand what's happening to her body? How is she going to understand the importance of good nutrition and generally keeping healthy during pregnancy? How is she going to make a birth plan? How is she going to cope with labour? Then, probably worst of all, how is she going to cope with the removal of her baby - because obviously she's not going to be able to care for it - and how is the baby going to be affected by being removed from its mother? (I was removed from my birth mother and adopted at 6 weeks old, in probably the least tragic set of circumstances possible as far as adoption is concerned, and even I am screwed up!).
Just so, so sad for all concerned