Very mild Crimes of Grindelwald spoilers
This is hardly a new idea, but it’s worth revisiting after Crimes of Grindelwald. I think it still works, perhaps even better than before.
First of all, Dumbledore seems to know a lot about obscurials. That could just be his tendency to provide whatever explanation the audience needs, but it could also confirm that he has past experience with them and, perhaps, a personal reason for obtaining that knowledge. Most wizards believe that obscurials no longer exist, but Dumbledore has all kinds of ideas about how their curse works and how it might be cured. He describes the obscurus as a “dark twin” born out of loneliness and says that finding his real family – a real brother or sister – might be enough to save Credence.
Obscurials typically don’t survive childhood, but Ariana was a teenager when she died. She didn’t even die from being an obscurial; she died when she was caught in the crossfire of a duel between the Dumbledore brothers and Grindelwald. You would think if Newt learned about obscurials from Dumbledore, he would have known that ten years old is not a hard limit and been less surprised when Credence turned out to be the obscurial in New York. However, Ariana was the exception. She was afraid of her magic and refused to use it, but she had a family who did their best to protect her, even if they didn’t fully understand what she was going through. Most obscurials don’t escape so easily from the circumstances that made them develop an obscurus in the first place, and most don’t have a loving family to watch over them. If Dumbledore did research after her death and found records that said most obscurials died before a certain age, he might have just assumed his sister was the exception.
Dumbledore does not seem surprised that Credence survived to young adulthood as an obscurial and suggests that he could be saved by the love of a family, which is exactly what kept Ariana alive for so long.