I'd like to know what the oldest mutually intelligible language would be. With each spoken and written language that exists, from how long ago could you find a

I'd like to know what the oldest mutually intelligible language would be. With each spoken and written language that exists, from how long ago could you find a speaker (or reader) who could meaningfully have a conversation with an average fluent speaker from today. I specifically say average, because vocabulary registers shift with time - even when fundamentals of the language do not, even an english speaker from after the great vowel shift might have trouble unless the modern speaker was especially well read.