In the U.S. it is usually late 20s or early 30s. People in rural areas tend to get married sooner whereas people in cities wait longer. You can get married at 18 without parental permission and in most states if you are 16 or 17 you need parental permission.

In Australia it is 29, but it depends on people's income and education. Here's a chart for the whole world http://en.wikipedia.org/wiki/Age_at_first_marriage

18 year old does not need permission but 17 can marry with permission (Ithink)

The Russians can get married from the age of 18 but in some cases like pregnancy from 16. In some Russian regions local government have an ability to decrease it, so that in my region it's allowed to set up a family from 16 and from 14 in before-mentioned cases.

In average Iranian get married in 24 to 26.