The verses, if not original, are known to be early because already in the 100's A.D. there are quotations of Mark's long ending. Irenaeus quoted the long ending around the year 180 in his book Against Heresies (III:10). Tatian seems to have incorporated the long ending of Mark into the Diatessaron (LV), which was probably written by around 160 A.D.
As it turns out, we do have one note passed down to us from earlier ages about who wrote the long ending to Mark.
An Armenian manuscript of the Gospels, dated 986, discovered by F. C. Conybeare in 1891, attributes Mark xvi:9-20 (the 'long ending') to the 'elder Aristion'. This may refer to Papias' Aristion, and may be correct. ("Who's Who in Eusebius" appendix, from entry on Aristion, Penguin Classics Edition of Eusebius' History of the Church, 1965, new editorial material © Andrew Louth, 1989).By its mention in this footnote, I have to assume that this information has been known to professional scholars of church history for some time, though it has not yet crossed over into common knowledge.
Even amateur students of church history will have met Aristion before, though possibly without recognizing or taking special note of his name. The early church leader Papias made some comments about where he received his information, and he lists some names. Among a series of names that are very familiar to us, we see the one unfamiliar name, Aristion:
And whenever anyone came who had been a follower of the presbyters, I inquired into the words of the presbyters, what Andrew or Peter had said, or Philip or Thomas or James or John or Matthew, or any other disciple of the Lord, and what Aristion and the presbyter John, disciples of the Lord, were still saying. For I did not imagine that things out of books would help me as much as the utterances of a living and abiding voice. (Quote of Papias preserved by Eusebius in History III:39, introduced by Eusebius as originally occurring in Papias' preface to his work The Sayings of the Lord Explained.)Papias lived and wrote in the first half of the second century; that is to say, this quote mentioning Aristion is probably not later than 150 A.D. In this quote, Aristion is identified as someone who had himself known Christ. Is it possible that he wrote the ending to the Gospel of Mark?
The timing is right: Aristion lived early enough to have made this addition to the Gospel of Mark in time to be quoted in the 100's A.D. He seems to have outlived many of Jesus' other disciples; Papias refers to what various other disciples used to say but to what Aristion was still saying, implying his survival at a later date. The circle of personal connections is right: Aristion had known Jesus and, judging by the honorable company in which he is listed by Papias, he was well respected in early church. And then there is the matter of the ancient manuscript which names him as the author: there is no apparent reason to have attributed that text to Aristion except for the obvious reason: whoever originally inserted that comment into the text had reason to believe that it came from Aristion.
Speaking for myself, one comment in one manuscript that is centuries removed from the original information is not enough to achieve complete certainty. But the information does fit, and unless further information comes to light I would accept Aristion as the probable author of Mark's long ending.
I've tracked down some more information, for those interested. The discovery by Conybeare had a writeup in the October 1893 (yes, 1893) issue of the Expositor, according to the references I can find. There were some subsequent discussions in the Expositor as to whether the person referenced was Aristion (from Papias) or Ariston of Pella, also of about the right time period. The name in the text is 'Ariston', though 'Ariston' was a common misspelling of 'Aristion' they say, and nobody was willing to put a huge amount of weight on the spelling of the name rather than just probabilities as to which one would more likely do that kind of work. The arguments that I read seemed to be leaning somewhat towards Papias' Aristion, though possibly with someone else doing the actual penmanship. I think the scholars studying the material got as much as they possibly could (or even more) from the one-liner in the manuscript about the origin of that section. I did see some images of the manuscript on-line (though not of the page of interest here). The manuscript seems to be called the "Etschmiadzin manuscript". So I'd probably give it 60/40 of being Aristion at this point, just based on the arguments I've heard, with the other 40 going to Ariston of Pella.