How can a person live that long without knowing their own date of birth in a developed country? Wouldn't he have needed to enter it in forms and such? He wouldn't even know his own age without a significant error margin.
@JohnnyC He might be guilty of fraud, then. There's no way the government wouldn't know his real date of birth. Unless Japan doesn't keep track of its citizens, which I'd find extremely hard to believe from such a highly developed country. He would have just needed to ask, although it's really in the realm of fiction he didn't learn it, like SquigglesJP said, whether he wanted or not. Id papers/passport would also state it.