This is a question that will never be conclusively answered. I have read quite a bit on this topic and the answers are all over the place. I tend to believe that the ending was original to St. Mark’s Gospel and is still quite ancient. The reason I think it is still quite old is because Mark chapter 16 includes a very brief reference to Jesus’s disciples on the road to Emmaus: " After this he appeared in another form to two of them, as they were walking into the country." (Mark 16:12).
Now, I believe that St. Luke’s Gospel was written no later than the mid-60s. But even going with the “modern” scholarly consensus that it was written in the 80s allows for an ancient pedigree of Mark 16. Most scholars who claim that Mark 16 was an addition say that it was added sometime in the very late 1st or early 2nd century. But these same scholars also claim that Matthew and Luke wrote after Mark, but added on and enhanced many of the stories.
If this is the case, why would an author adding Mark 16, clearly with knowledge of the Emmaus passage from Luke, give it such short treatment? This flies in the face of the scholars who claim that in every other instance, an author writing later than another will tend to add to the stories. But Mark 16:12 is just one line, very off-handedly referencing the Emmaus encounter. Luke writes about Emmaus at length - over 20 verses!
In my opinion, the comparison of the Emmaus encounter between Mark and Luke gives clear proof that the entirety of Mark was written first. And if Luke was written in the mid-60s, I think Mark can easily be placed in the late 50s - early 60s.