23 words

According to a research team, these words have been around for 15,000 years:

Listed by the number of language families in which they have cognates.

7 - thou
6 - I
5 - not, that, we, to give, who
4 - this, what, man/male, ye, old, mother, to hear, hand, fire ,to pull, black, to flow, bark, ashes, to spit, worm

Here's a link from the article at The Washington Post Health & Science section. And, from this page:

A research team led by Mark Pagel at the University of Reading in England has identified 23 “ultraconserved words” that have remained largely unchanged for 15,000 years. Words that sound and mean the same thing in different languages are called “cognates”. These are five words that have cognates in at least four of the seven Eurasiatic language families. Those languages, about 700 in all, are spoken in an area extending from the British Isles to western China and from the Arctic to southern India. Only one word, “thou” (the singular form of “you”), has a cognate in all seven families.