I'm working on a NPC who was raised to believe he is half elf, half dragon (Wood Elf father, Red Dragon mother), but who is actually a full Copper Dragon. The character is supposed to be 26 years old, and when shapechanged into an elf he has the appearance of a 26 year old elf (an adult). Most importantly the character gets married in his late teens or early 20s. The timeline of his birth and marriage are important to the story.
My question is, how does dragon age work with shapechanging in 5e? If being 26 years old makes him a young dragon, does that make him a child, a teenager, or a young adult? Does it make sense that he's shapechanging into an adult elf? If he gets married at the age of 18 to an 18 year old elf, is there anything ethically wrong with that (did a child just marry an adult)? If he shapechanges at the age of 5 as a wyrmling into a 5 year old elf, then shapechanges at the age of 7 as a young dragon does he shapechange into a 7 year old elf? Then at again the age of 18 an 18 year old elf?
I'm starting to think actually making him a half-elf would be easier because then I could say he matured at the rate of an elf.
Best Answer
Dragons change their physical characteristics, but not their cognitive capabilities.
Ancient Copper Dragons have the ability to change their shape (MM 110 edited for clarity) Dragons in general (MM 84-118) only gain the ability to change shape at the age category (MM 86) adult or ancient depending on their colour:
The physical age appearances of the new body are not tied to the shapechanger, but the shapechanger retains their cognitive capabilities. To be a dragon capable of Change Shape this preassumes an age of 101+ years, so the character is always an adult by the book and well suited to marry an elf that assumes adulthood at around 100 years (PHB 29).
If you decide to give the Young Copper Dragon (MM 111) the ability to Change Shape, then this would be a marriage between entities which are considered not adult but would both have a physically mature body (PHB 23, 29), if the dragon chooses to assume the shape of a physically mature body. As to say it would be two non-adults marrying each other by political necessity.
There is no by the book basis that makes Dragon life stages comparable to human social categories like a toddler, pre-schooler, child, etc. By the book, dragons do not adhere to these categories.
You are the DM and can make it up as you see fit. If you want to get some inspiration (including player character dragons), then you might want to read Draconomicon: The Book of Dragons which is for 3e and 3.5e