After epically stepping out of the Crystal Cave, Merlin looked down at himself. When, how, and why did he change into his Dragoon the Great outfit and age himself to be 80?

Morgana already knew he was Emrys, so the only reason to keep his identity hidden was Arthur–and, well, the rest of Camelot, but still. The aging spell was exhausting, being old meant he would be slower in getting there, and sure he was Emrys but there were prophecies to defy and a lot more to fulfill.

He changed back to his normal youthful self.

Ah, much better.