When was the English language started?

There are two points of view on this.

Cambridge University considers the English language to begin when Anglo Saxon (the West Germanic language which came to Britain in the fifth century with the Angles and Saxons) began to lose its case endings and absorb large amounts of Norman French and Norse vocabulary - sometime in the Eleventh century. From a Cambridge perspective English is a language which develops from a mixture of Anglo Saxon, Norse and Norman French sometime after 1200. This makes English about eight hundred years old.

Oxford University considers Anglo Saxon to be the same language as modern English, and calls the language Old English in fact. So from the Oxford perspective the language of Beowulf (composed before 800) is already English (Old English). That point of view makes English over a thousand years old.

Both Universities agree that Modern English begins sometime around the Fourteenth Century (Chaucer writes in Middle English) and is recognisably the language we speak today by about 1550 (Early Modern English begins around the time of Edmund Spenser (born 1552).

So it depends on your point of view really.