My atoms are 13 billion years old or whatever it is. I guess you, me, and water are all the same age. After we die and desiccate our water will be joined to other fremen. Wait..
The 7 in pH 7 meaning neutral comes from 1 in 1e7 molecules of water having broken apart and ionized at any given instant. Like the Taylor Swift song says, they're never ever getting back together. Ever. So the average lifespan of a rando H-O-H thrupple in plain old tap water is about half a day. So by the definition of "hydrogen atom with UUID X" in a monogamous bond with "oxy atom with UUID Y" in a monogamous bond with "another hydrogen atom with UUID Z" that relationship on average lasts about half a day before getting stirred up into a new relationship. In that sense, most water is less than a day old.
The article is defining age of water as in how long it was sealed off from the rest of the ecosystem. In that sense I have a bottle of Crown Royale containing about 60% water in my basement that's been sealed off from the environment, theoretically, for 18 years before it hit the liquor store shelf, or so the marketing claims. Actually a little longer now that it's chilling in my basement. These folks found some water that's been sealed off for a billion years.
So there's decent arguments for both that "a bucket has been sealed for a billion years" vs "this particular organization of atoms has existed substantially less than one day"
I think the title here is failing to be an accurate description, unless I missed something in science class.