[Supperman]

OK.
The matter is much complicated as I first thought.

I looked at my Yahoo.Japan account. And found that my birth-date is -非表示-,too.
It is not my decision, but Yahoo Japan's.
Yahoo Japan prevents my birth-date to be shown to the public, nor myself....

I tried to check my Birth-date information, but I failed.

If you pretend to forget your password, then Yahoo Japan asks you about your birth-date. So if you type your birth-date, and if your birth-date typed and the information of your birth-date in Yahoo's computer match, you will know that your birth-date information is correctly reserved in Yahoo's computer.