Most people have lost their first baby tooth by age 7, and girls lose their baby teeth earlier than boys. I'm a female, and I remember losing my first tooth aged 6, and I was the last of all my friends, male or female. From this, I think it's very unlikely that this Doe was 8 years old.
I agree with the above comments about gender identity; it is not appropriate to be inserting gender politics into the murder of a child. Especially not as there is no doubt about her sex, and the fact that she was presenting as female, therefore gender politics is irrelevant to giving her her name back.