Robert M. April was a name Gene Roddenberry used for the main character of a detective show or two or three or so that he wanted to get on the air and failed to.
He simply recycled the name as the Captain of the Yorktown/Enterprise. He changed the middle initial and based the character as a 34-year old Captain. The Star Trek story was to be 300 years in the future, which at the time was 2264. Ultimately for some reason the name Robert T. April was dropped, and instead Captain Pike was created, which was played by, then, 38-year old Jeffrey Hunter for the first Pilot-Episode, which was to take place as before, 300 years in the future.
Since "The Cage" was a flop, and Jeffrey Hunter didn't want to have anything to do with Star Trek so it would seem, they needed a new Captain, which they found in William Shatner, who happened to be about the same age as the initial Robert April Character (Shatner was 33 in 1964).
Since the story took place in 2264, which was the same time frame that Robert April was supposed to have the Enterprise, and the same time that Pike was supposed to have the Enterprise, Robert April was written in as the ship's first Captain, and Chris Pike, as the second. It would seem since Captain Kirk was about the same age as Robert April was supposed to be, so he became the 34-year old Captain, who was said to be the youngest in history. This seems to have automatically required that Robert T. April would be an older guy.
The fact that Gene Roddenberry (who was 43 in 1964) was depicted as Captain Robert T. April (which was probably purely homage to Mr. Roddenberry, who had died, at the time, not too long ago) actually confirms this.
I would say that Captain Robert T. April is canon...
CuttingEdge100