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|| This article contains fan speculation.|
There is no solid evidence for or against some parts of this article.
The aging of characters in the anime has been a subject of common fan debate, since it is not addressed by the anime itself. The passage of time is clear, but characters show small, if any, signs of aging. The writers and character designers seem to ignore the issue of aging, usually so the viewers of the show feel closer to the protagonists, as it is easier for a child to relate to a ten year old rather than a twenty year old.
According to the eternal youth theory, like in many cartoons, the characters never age and are trapped at the age of their first appearance. This also suggests that the show has a floating timeline.
This is the explanation given in the Pokémon.com mailbag for why Ash is still ten.
Ash Ketchum was indeed ten years old when he started, but he is ageless – since the beginning of the show until now, Ash has become an icon, a legend, like Mickey Mouse. But Mickey Mouse doesn’t look 75 years old (although he is), and the same goes for Ash.
However, this seems strange since the passage of time is pointed out every once in a while without the mention of any character's birthday. The dub has mentioned large periods of time passing, despite Ash still being 10 years old in In The Shadow of Zekrom!. One of the most common supports for this theory is that Ash and May are supposedly the same age when May begins her journey, even though Ash has traveled through two regions and competed in three leagues before she received her first Pokémon. This is also the case when Dawn starts her journey as she is the same age as Ash, even though he has been through four leagues by this time, as well as traveling through Kanto twice.
In the English dub, many characters have had their voice change over time, especially Ash, whose voice has become noticeably deeper over the seasons, giving the impression of aging. Brock and his family are often used as evidence towards this idea as Brock should always be five years older than 10-year-old Ash and his youngest siblings barely look any older between Showdown in Pewter City and DPS02.
According to the slowed time theory, because only one half-hour episode airs approximately every week, the flow of time in the anime is considerably slower than in real life. This theory is supported by the apparent, though minimal, growth of the characters, as well as the noted marks of time.
However, this theory still faces the same problems as the Eternal Youth theory.
According to the arbitrary age theory, the label of age has a different significance in the Pokémon world than in the real world. In their culture, the numerical "age" of a person is not representative of the number of years which they have lived, but representative of the person's familial status. When a person comes of age and is ready to head out into the world of Pokémon, they are labeled "ten." When a person has children, they are labeled "twenty." When they have grandchildren, "fifty." Other ages may exist but there is not enough evidence to deduce these ages and milestones in one's life. The theory argues that these seemingly young ages for things like having (grand)children are similar to the surprisingly young age at which children come of age. Ash may well have been alive eighteen years, but hadn't earned the position of more than "ten." Brock's familial role is most accurately described with a half-age, "fifteen". He doesn't have children of his own but he takes care of his younger siblings in the absence of his parents.