Ten years old is exactly what it says. One point though it is not ten years from date of japan registration it is ten years from date of manufacture which is usually earlier in some cases by 3 or 4 months if the car was not sold for a time.
It is done monthly because most japanese export certificates only give month not day of registration . If you time the application right the registration docs can be issued on the first of the month
Also if you can get the exact date of manufacture confirmed by the company (eg nissan) on official paper for example the 15th you can register it from the 16th but you have to pay for the whole months tax.