The English language has changed over time - if you had a time machine, how far back could you go and still understand English? At what point in history would you not be able to understand?
If you went back in time to the 1800's and 1700's, you'd probably still be okay. This except is from the book 'Robinson Crusoe', in 1719:
"I, poor miserable Robinson Crusoe,
being shipwrecked during a dreadful storm in the offing,
came on shore on this dismal, unfortunate island,
which I called “The Island of Despair”
That's fairly easy to understand, but you might struggle with old slang words like 'batty fang' and 'kickerapoo'.
batty fang - a beating
kickerapoo - dead
land pirates - highway robbers
gutfoundered - very hungry
whapper - a big lie
Nitsqueeger - Hairdresser
Xantippe - an ill tempered wife
Abbess - a nun
Thornback - a spinster
Barber-monger - a vain man
Bleater - someone who complains a lot
Brabble - to quarrel loudly
Crapulous - the feeling of being too full
Hugger-mugger - secretly
Lettice-cap - a medical device like a hair net
Pigarlik - a bald head
Petty fogger - a dodgy lawyer
Mumpsimus - the act of sticking to old mistaken beliefs about language and customs simply out of habit
The 1600s is the time of Shakespeare.
"Thy natural magic and dire property,
On wholesome life usurp immediately."
Here, Lucianus talking about the natural magic of poison. and how using it to kill the king will usurp the throne.
Trickier to follow, but not everyone spoke like that. You would hear lots of words you didn't understand though.
The bigger problem for you now is the pronunciation. The sound of the vowels has changed, and the accent is becoming much harder to understand.
For example, "tea" is pronounced "tay", and "gone" is pronounced "goan".
In the 1500s people essentially speak like the Bible.
"Now therefore thus saith the Lord,
Thou shalt not come down
from that bed on which thou
art gone up, but shalt surely die."
KJV 2 Kings 1:4
There are also hundreds of words that don't mean a thing to you.
Before about 1400 AD, you'd hear Middle English, and you would hardly understand anything, written or spoken.
"Whan that Aprill, with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour;"
The Canterbury Tales, 1389
If you went all the way back to one 1000 AD, you'd hear Old English:
Fæder ure þu þe eart on heofonum;
Si þin nama gehalgod
to becume þin rice
gewurþe ðin willa
on eorðan swa swa
Good luck explaining that you need 'one point twenty-one Jigawatts' to get home!