Most European languages include both formal and informal second-person pronouns. (They also gnerally include both singular and plural pronouns.) Formal pronouns are used when speaking to someone you don't yet well enough to use informal pronouns with.
The definition of "knowing someone well enough to use informal pronouns with them" has greatly evolved over the years in all of these languages. In books from the beginning of the 20th century and earlier, it is common to find characters who are married, often for many years, and still use the formal "you." Note that this does not mean that the people involved do not like each other -- the formal "you" is also seen as a sign of respect.
AFAIK, the informal pronoun has always been used, in most languages making this distinction, for parents speaking to their children.
Recently I have been reading a Western by the GermanKarl May, written in the 20's, in an edition translated into Czech in the mid-70's. Despite many other language changes in that time, the formal pronoun is still used in conversation between (male) characters that work and play together (you know, that cowboy stuff) closely for many months.
When I was learning French in the mid-80's, my 40- or 50-year-old teacher insisted that a good rule of thumb for a minimum period of acquaintance before using the informal pronoun was 7 years. Seven years!
However, among less cultured people like most people I meet in life, the informal pronoun is acceptable after a few weeks or months at most, and is a matter of intuition. Students and youth generally use the informal pronoun for each other right away. The most probable cause of these changes, in my opinion, is the general reduction of formality in relationships throughout industrial societies (even communist ones) in the last 50 years. I guess there is something about industrialization that does that to a society.
It can be hard for people from the United States to learn the ins and outs of using these pronouns, as we are a very informal society. I find myself tending to use the informal pronoun by mistake quite often.