At what age in 1930 was it legal to have sex?

Depends on where in the world you lived.  Some nations were being to write laws to protect children.  Others were not.  Even today the age of consent very from 12 to 20, with some countries not having any laws on the subject.

Prior to the first written laws, the age was usually once a girl had begun to have her monthly "cycle."  That can vary depending on the individual.  So it was not based on an "age", but on their physical maturity.  In many culture there were no "birth certificates" so often a person's age was not even known.

In the United States, the age was 10 to 12 (varying by state) when it was first founded.  In the 1870's England had raised the age to 13.  The US follow shortly there after.  By the 1920's the only state in American that had an age of consent below 16 was Georgian (at 14). There was no age of consent for boys.

In which country?

Information on the ages used historically in western age of consent laws is not readily available. This table has been compiled from a combination of historical and contemporary sources. By 1880, the first date chosen, many western nations had established an age of consent for the first time, typically of 12 or 13 years. By 1920, when the influence of reform campaigns that established a new link between the age of consent and prostitution had run its course, most had revised their age upward, to 14 or 15 in European nations, and 16 in the Anglo-American world. In the last decades of the 20th century, states and nations with ages below those averages amended their laws to move closer to them. In Europe that growing conformity owed much to moves toward greater European integration. Given that the rationale for the age of consent has remained essentially unchanged in its emphasis on the need to protect 'immature' children, the table highlights the shifting and various definitions of childhood employed across time and cultures.