In Japan, its 1.4 years (pdf). Thats a pattern seen around the world: Age gaps show up among heterosexual couples in every culture. Some evolutionary explanations have been proposed (older men may have more resources, younger women may be more fertile), but none are easily testable.

What age do Japanese men marry?

Mean age of marriage Japan 1955-2019, by gender In 2019, it was estimated that the average age of women who marry for the first time was 29.6 years, while men were on average 31.2 years old when they first got married.