TOKYO (AFP) - Japan passed legislation to lower the voting age to 18 from its current 20 on Wednesday, allowing teenagers into polling booths for the first time.

The move will bring Japan - where political power resides firmly with the burgeoning older generations - into line with other developed countries and will extend the franchise to an extra 2.4 million 18- and 19-year-olds.

These young people are expected to cast their first votes in the upper house election scheduled for summer 2016, unless the lower house is dissolved for a snap election before then.

The bill to revise the public office election law passed the upper house after being approved in the lower chamber.

Japan last changed its voting rules in the punch-drunk months after its 1945 surrender in World War II, altering the age at which citizens could cast their ballot from 25 to 20.

Around a quarter of Japan's 127-million population is aged 65 or over, a result of low birth rates over the last few decades and no significant immigration. The proportion is expected to grow to around 40 per cent in a few decades.

The inverted age pyramid that this represents - combined with a Confucian respect for elders - has left Japan a country primarily run by, and for the benefit of, old people.