I like that Isaac comes across as this creepy crazy midget in this film.
You've seen Guy Standeven in something because the man was in everything.

I think both theories make sense. It does add a layer of creepiness that we're not sure what exactly is Isaac's backstory in the film.
So many stories, so little time.

I see your point, but children can seem older than they are. When I was growing up I talked like an adult and that had to do with the fact I wasn't talked to by my family like I was a child and I believe that was the case with Isaac as well.
faith begins at the end of your comfort zone.

I have read the short story and know that in it Isaac is nine. However, Job's line suggests that Isaac is not a young person. A kid would refer to the early or middle childhood of another kid as the time when someone is 'little' or 'small', not 'young'. Isaac has a highly manipulative, authoritative air that a child, even an unstable one, would not bear. He carries a fervor for wholly controlling lives whereas the original version presents the preacher as one who simply wants to be agreed with and followed solely for religious reasons. Isaac holds sophistication that he lacks in his debut in which he seems closer to an evangelist than a cult leader. Also, the movie is very different from its predecessor. Perhaps the film gives Isaac's character an update to accommodate a grown actor. I realize that we cannot be sure of the age of Isaac, but strongly get the impression that he is an adult.

In the short story Isaac is a child preacher.
So I think they mean "when he was young." To mean like 7. And he's slightly older now (like teenager)
I wouldn't read too much into it.