Pre-Bat Mitzvah - still old enough to decide

The Daily Telegraph reports that a court in England (Essex) has ruled that a Jewish girl of 10 is old enough to decide that she wants to convert to Christianity.

Apparently her Jewish parents divorced, and her father converted to become a Christian. Now the girl wants to join the father in his religion, against the wishes of her mother and grandparents.

The judge has a job to do, and can only base it on the evidence which he is presented with. He obviously cannot tell her to wait until she is 12 in keeping with halacha.

I wonder how much of a Jewish education and Jewish homelife the girl received from her mother and grandparents? And what is so special about the age 16 when they feel that she would be able to decide for herself? Is there a Jewish source for 16? If not, haven't they just lost their own argument? If they would have said 20 it would have made more sense to me. Though I think in Judaism from the age of 12 her decision should be valid.

But the most interesting part of the article is the letter which the Rabbi wrote, explaining that converting will be damaging to the girl's soul. In the judges words, the rabbi’s letter was made in “inflammatory terms without any supporting evidence”. I wonder what sort of supporting evidence the judge would have accepted. How does one prove the effect on a soul? On the other hand, why did the Rabbi decide to write in terms which the judge considered 'inflammatory'? Could the same letter not have been written in a nice way?

Actually, I just found a report on the Jewish Chronicle website. This article names the Rabbi, and quotes part of his letter:

In Judaism we don't encourage conversion either way as it is unnatural for a person to change the religion they are born into and which thus is ingrained in their soul in a deep way. Although conversions are performed they must be worked at over a number of years when a real change can realistically take place. It is unfair to any child to put them under this pressure and to do something unnatural to their soul.

Is that the inflammatory bit? If so, I really don't understand the judge. I can see why such a letter would not be helpful or persuasive, but I don't see why it is inflammatory.