man is it ever impossible, but okay I’ll try to summarize my headcanons!
Just based on when the characters were invented, I think Fethry is the youngest cousin, Gladstone is the middle cousin, and Donald/Della were the oldest. For some reason I think Gus is about the same age as Gladstone? But less sure on that. Because Gladstone and Donald are often shown in school as being in the same year/about the same size as kids I think they’re pretty similar in age, so like a year apart tops. Fethry I see as being one-to-three years younger than Gladstone because he’s so zany (and is definitely a more recently invented character). I’m not sure about Whitewater but just because he’s so scrappy and energetic perhaps he’s between Fethry and Donald/Gladstone’s age?
In terms of general age brackets I’d say Donald and Gladstone are in their mid/early thirties (considering Huey, Dewey and Louie’s age is probably about 10 in the comics, and I imagine Della having them when she was in her early twenties), and so Fethry might be in his late twenties or just hitting 30. Magica I always saw as being in her mid/early 40′s, Scrooge in his 80′s and Grandma is anyone’s guess man that girl is indestructible.
iNCREDIBLE ADDITION ACCEPTED