This is true of some languages which have a high rate of change, such as English, but much less true of others. As a native Persian speaker for example, I can read the Shahnameh (Book of Kings) of Ferdowsi, which was written around 980 (very much contemporary of Beowulf), and understand about 95% of it. Nearly every literate Iranian would be able to read it without an issue, and at most modern prints have footnotes to explain the words which have fallen out of use.

Not every language changes at the rate English does.