How old is English Language?

very debateable - some words still survive from the Celts (500 BC) but English as a language didn't become official until October 1362. The earliest document that still survives in English is dated 1258 (known as the Oxford Provision)

Several sites quote 450 AD. The following is probably the best explanation.

'English originated in England. Before that, the languages ancestral to
English were spoken on or near the coast of northwest Europe: lowland
northern Germany, southern Denmark, the Netherlands. Some linguists refer to
these languages as 'West Germanic.' Today, the language closest to English,
in both its history and its structure, is Frisian, spoken in the northern
parts of the Netherlands and in those regions of Germany closest to Frisia.

With the coming of the Angles, Saxons, and Jutes to England, beginning around
450 a.d., the process of separating English from other West Germanic
languages had begun.'

[From Linguistlist.org 1998 article, no longer online]

See also:

'Dating is not easy, but written records and archaeological evidence indicate that the island of Britain was settled by speakers of the ancestor of the English language around the year 450 AD.'

[From Vcsun.org article, no longer online]

So great has been the evolution of the language that we would be unable to understand what they were saying back in 450!

English is normally divided into three languages: Old English, the language of the poem Beowulf, was spoken from about 500 AD to about 1000AD. Middle English, the language of Chaucer and the Canterbury tales was spoken from about 1000 AD to about 1500 AD. Modern English was from about 1500 onwards. Most of us can understand Shakespeare with a bit of effort. He wrote in Modern ENglish. Middle English and Old English are so different that they could be considered separate languages. Here's the first verse of Beowulf as an example:

hwaet we gar-dena in geardagum,
theodcyninga thrym gefrunon,
hu tha ae%FEelingas ellen fremedon.
oft scyld scefing sceathna threatum,
monegum maegthum meodosetla ofteah,
egsode eorlas, syththan aerest wearth
feasceaft {funden;} he thaes frofre gebad,
weox under wolcnum, weorthmyndum thah,
oththaet him aeghwylc thaer ymbsittendra
ofer hronrade hyran scolde,
gomban gyldan. thaet waes god cyning!

Some more details about the earliest stages of the English language: https://en.wikipedia.org/wiki/English_language#History