For decades, people have wondered how the Disney ducks characters are still alive and well when, if we count from the probably birth dates given in comics, cartoons and interviews, Scrooge McDuck must be 148 years old, Donald Duck approximately 90, and Huey, Dewey and Louie 75-80. However, most modern comics still present our beloved characters as unchanged since the 50's, with no or little canon explanation.
Don Rosa, to get around this problem while writing Duck stories during the 90's, said that all his stories were throwbacks to the fifties and sixties, but that only works for him; DuckTales or numerous comics either mention a date such as 1994 out loud, or make use of technology that obviously didn't exist in the fifties, such as cellphones or computers. There has been another attempt, by other auhtors (mostly Italians) to apply to Duck comics the same principle used in The Simpsons or Captain America, the Intemporal Chronology, in which the amount of time between "present day" and past events of a character's backstory shifts along with the year; admitting Scrooge was supposed to have been born in 1867 in his first appearance in 1947, this theory would retcon that in the "current" stories, he is supposed to have been born eighty years ago from today, that is in 1936. This doesn't work either, because of those very same stories that clearly mention dates (in a few decades when it'll have been retconned that Scrooge was born in 1954, how are they going to work out a story made in 1954 in which a character states the date ?), and also because some things just can't be moved (Scrooge just can't have made his fortune during the Klondike gold rush in the fifties, that doesn't make sense, but him having gotten rich in the 1896 Klongike gold rush is one of the few things that no author has argued against since they were implemanted into the canon by DuckTales and Don Rosa).
So, it doesn't work, sorry.
2° A little glow in the background ?
3° The glow strengthens
In Don Rosa's twelvth installment of The Life and Times of Scrooge McDuck, it's hinted that a character's state of mind has something to do with his physical state; as soon as Scrooge goes back on adventures, his wrinkles disappear and he says to himself he "feels young again".
4° Willpower will keep you alive's pilot The Valley of the Golden Suns. There is, in it, one character that might be a beginning of an answer: El Capitan. His entire gimmick is that he is a spanish conquistador who saw a valley full of riches, but had to leave it primaturely and without a map, and became obsessed with finding it again. The incredibly strong will he had to find the treasure allowed him to stay alive for four hundreds years, always searching for the treasure or some way to retrieve the map he had lost. When asked how he lived that long, he simply answers « Sheer willpower ! ». Aside from the wild theory that El Capitan actually found the fountain of youth while searching for the Valley, but lies about it because he's so protective of everything he finds, I think it is a pretty solid explanation. It has been guessed that Scrooge and Glomgold are also affected by the same thing as El Capitain: they both love their riches so much that they can't bear leaving them. But so far it only works on McDuck and Glomgold, with gold. Not anything else.
5° Eureka ! Canon all along !
(Yes, I tweaked the order, the story I'm going to talk about now is slightly older than DuckTales, but it is also much lesser known, so here you go)
Then comes Carl Barks's illustrated tale Go Slowly, Sands of Time. In this short story he made while he was already retired, Scrooge begins to wonder what shall happen to his fortune when he's no more part of this world. Dreaded at the prospect of what Donald or the Duckburg municipality would do with it (that is: SPENDING it. What a horror !), he decides to go on a quest to find a way to live forever. He goes to a valley in Himalaya where people are known to live very long, and tries to eat of many things that are available there, hoping that one of them is the reason of the people's near-immortality. As it turns out, none is, but… the reason they live so long… is that… they enjoy… doing…what they do… everyday. Starting with the centuries-old town treasurer, who swims everyday in the town money. You get the picture: Scrooge already had the secret of immortality back home ! And it's swimming in his money !
And the interesting point is, Carl Barks purposedly did this story as an explanation for the characters's extended lifespans:
"Go Slowly, Sands of Time! is kind of a spiritual story of Scrooge, which gives him a means of going on and on forever so you don't have to feel that he comes to the end of the line and dies all at once. He's going to just keep on going into eternity. Kids that read about him a hundred years from now can think of him as still being alive. He has found the fountain of everlasting youth with his money."
See ? That's exactly how it works — like El Capitan, madness aside: in the Duckverse, as long as you've got something you're doing often enough that you love doing, you're going to stay alive forever or almost so (unless they die of injury or illness). The book mainly talks about Scrooge, but the ending hints that Donald's own immortality fountain is a good ice-cream-soda.
As for Huey, Dewey and Louie Duck not aging in all these decades… My guess is that they fully realize themselves as Junior Woodchucks. They don't want to grow up, so that they can be Junior Woodchucks forever. As it turns out, willpower might slow down any aging — including the one that turns children into teens and teens into adults. (Or they're just born with a genetic disability that stopped their physical development at a young age — a Duck equivalent to dwarfism, if you will —, if you feel less poetic, but I prefer my first guess.)