For the most part, it IS true. Anything built before ~1982 is too dangerous because of the standards upgrade you mentioned (1981 isn't safe enough: houses take

For the most part, it IS true. Anything built before ~1982 is too dangerous because of the standards upgrade you mentioned (1981 isn't safe enough: houses take some time to construct, so some from that year don't meet those standards because they were started earlier), but there was another standards upgrade around ~2001 (because of the '95 earthquake in Kobe), so homes built after that time are yet another step higher in safety. Another factor is renovations: many buildings go through scheduled renovations after 20 years, which can be disruptive (you have to move out temporarily). A newer building is farther from that point.