When Honda Motor Co. launched the latest version of its N-Box a year ago, it promoted features on the pint-sized minicar such as error-detecting pedals, automatic emergency braking and moveable seats, part of a push to market the vehicle to young families.

But a drastically different demographic has made the N-Box the country's best-selling passenger vehicle: roughly half the owners of the most recent model are 50 or older.

Automakers had hoped high-tech options would attract younger buyers to kei cars (minicars) even as the number of Japanese drivers under 30 has slid nearly 40 percent since 2001.