Bicycle is a popular mean of transportation in Japan and is widely used by people of all ages. Most Japanese bicycles are not made for sports, but are rather geared for comfort with extra accessories like the popular front basket.
Road bicycles, which are commonly used by commuters in American or European cities and known for their low weight and high speed, are rather rare in Japan. Only race-oriented bicycle shops carry them and they are not often seen on the streets. The accessorized and comfortable commuter bikes are named Mamachari in Japanese and like their name suggests (mama is used to name moms in Japan), are often used by the latter to carry a whole family to school.
Interestingly, a lot of smaller, fold-able bicycles can be seen in Tokyo, probably due to the easy storage in Tokyo’s small apartments. I guess also that most train lines would accept those folding bicycles if they are folded and stored in an appropriate bag.