All diamonds, as far as we know, are quite old on the Earth. Most diamond formations probably took place on the Earth in the first couple of billion years of the Earths history. There are diamond deposits that have been discovered that are youngerthe rock itself, the Kimberlite, is maybe just tens of hundreds of millions of years old. The way they date diamonds is typically looking at inclusions of other minerals in the diamond that can be radioactively dated. The diamonds themselves cant be dated. But if the mineral inclusions contain certain elements like potassium and things that can be used in a radioactive dating scheme, then by dating the inclusion in the diamond you get some sense of the age of the diamond itself. And those dates always suggest the diamonds are quite old. At least hundreds of millions of years old, but in most cases, billions of years old, anywhere from one to three billion years old, a time when the earth was probably hotter than it is today and so conditions were perhaps more appropriate for diamond growth.