Production on this episode began long before the series premiered.
As mentioned above, this episode was meant to be the series premiere, but it was actually "DeeDeemensional" that aired first, as mentioned above. This episode was held off until the following week as the first part of the third episode.
"Old Man Dexter" is the third pilot of "Dexter's Laboratory". It was the only pilot that didn't air in "What a Cartoon!", because it was declared unaired in the USA.
This episode is also aired on May 11, 1996 on TNT as a sneak preview.
This episode seems to draw animation style comparisons to the first episode of Season 1, this would indicate that this was an old episode already completed before it was aired.