It depends on what you mean by the English language. The modern English language has a few main sources of vocabulary, which I'll go through chronologically. The inhabitants of Albion at first contact with recorded history probably spoke a group of Celtic languages. You then have a Roman invasion adding a little bit of Latin (for the first time - a second Latin wave will come later). In 449 a group of Germanic peoples called Angles and Saxons invaded Albion, bringing their Germanic dialects with them. The language spoken on Albion at this time is what can first be identified as "old English."

The next major addition came during the Norman conquest in the 11th century. During this era the upper classes continued speaking Norman French for quite some time, leading to a lot of French words entering the language. Throughout the entire medieval period more and more Latin also started getting adapted into the English Language through the clerical classes, leading to a second batch of Latinate vocabulary - which was much more significant than the original borrowing from Latin the late classical period.

So that's where the vocabulary from English came from, and from about 500 you could identify something as the "English Language"

Morphologically, however, the language changed very rapidly during the early modern period. Any modern English reader could read, with some difficulty, the English of Chaucer in the 14th century, though a lot of vocabulary and spelling would be unfamiliar. You could not, however, understand the spoken language, because the great vowel shift had not happened. "I see a knight" would be written like that, but pronounced (roughly) "Ee say ah kuhneeght." By the time you get to Shakespeare, however, just a few centuries later, the language (both spoken and written) would be relatively easily understandable for a modern English speaker/reader.

So to summarize - like all languages, "English" has its roots in the deep, pre-historic past, and can be traced through many centuries of development. If you're asking when you could look at the language as spoken and written, and knowing only modern English say "this is the same language" the answer is probably somewhere between the year 1300 and 1500.