It is hard to find specific details on the Ministry of Foreign Affairs website, as the only thing that mentions it is the very similar type O-X visa, which is just like the O-A retirement visa, except it is good for ten years, five years at a time, and it mentions that a wife of any age must meet the same requirements, but then on legal websites they state that yes, if the wife is over 50, then she must meet the same requirements. "The financial requirement will stay as it is for individual applications. Please take note, that if you and your wife are both aged 50 and above, each should meet the requirement, this requirement will not be presented as couples. Yet, if your wife has not reached the age of 50, she can act as your dependent."
*********************************************************************