With roots in the 9th century, the first written examples of something clearly distinct as “Croatian” appeared in the 14th century (in the form of prayer books, primarily). But the language didn’t enter into a ‘standardisation’ period until the 16th century. Standardisation sometimes has a distinct period that ends with a generally accepted language form, but in the case of Croatian history and politics have caused a second wave of standardisation to take place in the modern day.
The first period of standardisation in Croatian occurred in the late 16th century, beginning with the first Croatian dictionary written by Faust Vrančić. This extended into the 17th century as the ruling families that controlled the area attempted to promote a unified culture, partially through a unified approach to the language.
Their approach was to write in a mixture of the three standard dialects being used at the time, known as Chakavian, Kajkavian and Shtokavian. This became the standard form of language for the elite as the movement for cultural identity grew in support and energy.
That movement was stopped, however, after a disastrous attempt at revolt against the Hapsburg Holy Roman Emperor that resulted in a severe clampdown on the area, including the execution of many Croatian political leaders. Attempts to unify the language were abandoned and replaced by an Austrian-sponsored version called Neo-Shtokavian.
The movement towards a standardised Croatian was picked up in the 19th century and was largely successful in the adoption of Neo-Shtokavian as the standard form of Croatian, as it was the form most widely used at the time. In an effort to make the language as Slavic as possible a great many neologisms were introduced.
After years of dominance under the communist regime and nearly a century of being artificially combined into a nation called “Yugoslavia,” Croats and Serbs have established themselves as separate countries and this has been accompanied by a fierce sense of national identity and ethnic pride. As a result, despite the fact that Serbian and Croatian are more or less the same language there is a modern attempt to standardise each along their own lines, in effect to make them as separate as possible.
The effects of these efforts are so far mild, but in time these attitudes may succeed in sufficiently differentiating the two versions of Serbo-Croatian enough to classify them as dialects, and then perhaps, in time, as separate languages. Whether this is a laudable goal in a world that otherwise seems to be trending towards closer relations is a matter of opinion.