Old age
After 600, dragons are considered elders and like humans, their bodies are starting to degrade, as is the level of their magic. While the average life span is from 800 to a thousand years, the oldest dragons recorded are of age over 1500 years.

Just going to put that out there. It's part of Forsythe's sheet.
And there's no rule that says a spell to change appearance can't change apparent age.