(UPDATE) Kyodo News just reported it was a girl, age 16 to 18.
The first confirmed case of thyroid cancer in a child in Fukushima Prefecture was reported in September this year, after the 2nd screening test which was administered as a precaution.
Possibly the second one may have been found, according to Kahoku Shinpo. What's different in this case is that the child was immediately categorized as "C" in the 1st round of screening. Category C is "in need of immediate second testing", according to the Fukushima prefectural guideline.
From Kahoku Shinpo (11/17/2012):
One case of suspected thyroid cancer, Fukushima Prefecture to conduct further test immediately
Fukushima Prefecture has been conducting the thyroid testing of about 360,000 children under the age of 18 to understand the effect of radiation due to the Fukushima I Nuclear Power Plant accident. It was revealed on November 17 by the source that they found one child during the initial screening who may have cancer and is in need of immediate second testing.
The details will be reported in the meeting of "Fukushima Prefecture residents health management survey" committee on Novmeber 18.
Fukushima Medical University, who has been conducting the test, says "It took 4 years to find a thyroid cancer after the Chernobyl nuclear accident", and the possibility of relationship between the radiation [from the Fukushima accident] and the potential thyroid cancer is low. However, blood and tissue samples will be taken to determine whether it is a cancer or not.
There are three categories, A, B and C, in the initial screening depending on the size of nodules/cysts. The child in question was judged to be in the "C" category.
In the committee meeting in September, it was reported that one child under "B" category (no immediate action necessary but conduct the 2nd test just in case) was found with thyroid cancer. However, judging from the state of the cancer, the committee thinks the cancer had already been developing before the March 11, 2011 disaster and denies the causal relationship between the radiation from the nuclear accident and the cancer.
Fukushima Medical University says, "This is an unprecedented study of all children in Fukushima Prefecture. So it is expected that a fair number of children are found with thyroid cancer. We will carefully study the relationship between the radiation [and the thyroid cancer]."
According to the Fukushima prefectural government (as of September 2012), the testing schedule for the cities in Fukushima is as follows (English labels are mine):