Question: When did Dean find out about the supernatural?
In my experience, fans usually assume he knew from the time Mary died. But were we ever actually told or shown that that?
In the pilot, when we see Mary die, it doesn't look like Dean saw her on the ceiling. If that were the case, it would have been easy for John to not tell Dean that part. A couple of episodes (Dead in the Water, Man’s Best Friend with Benefits) do seem to imply that Dean did see Mary on the ceiling. Even if this is true, it seems like something that an impressionable child could easily be convinced didn't actually happen if his father wanted.
We know that Dean knows about monster by the events of Something Wicked. The transcripts deems him nine or ten at that time. We also know from All Hell Breaks Loose Part 2 that when Sam started asking questions at age five or younger, Dean knew the answers. Sam being five puts Dean at around nine. If Sam was a little younger, say three or four, Dean would have been about seven or eight. That's a far cry from the assumption that of course Dean found out at age four.
I can't think of any other information that we're given in the first ten seasons.
The transcript for AVSC puts Sam at eight and half when he discovered the truth. Dean could have been younger - but he could easily have been the same age, or even a little older.
We know how Sam found out the truth. How did Dean?