In Christianity sexual life outside marriage is banned and for ages it was like that in Europe and the Western world, I want to know at which age this start to became popular that people will have relationship and girlfriends instead of being married.
In the late sixties. This is called "sexual revolution", and some social scientists explain it by invention of reliable and convenient contraceptives ("the Pill"). Of course, decline in religion is also an important ingredient, but this process began much earlier, I mean in Western Europe.
In other parts of the world this happened later. For example in the countries of the former Soviet Union in the 1980-90. I mean sexual relations without marriage were quite common already in 1960-70, but they were not socially acceptable: people had to hide them. In the 1980-90 this became completely normal.