The Pokemon franchise, including the games and the anime series, has many unchanging mechanics at play. One of these mechanics includes the appearance of anime series protagonist Ash Ketchum, as seen in the trailer for the new Pokemon series and a tweet emphasizing Ash's appearance throughout each anime series.
Ash Ketchum has, by and large, been the focal point of the Pokemon cartoon shows since the late 1990s. There's been a revolving door of differing themes and supporting characters, which may soon include a character with a name referencing Pokemon GO. However, the point stands that Ash Ketchum has been the rock on which the anime series was built.
This point was emphasized by Twitter user @cota_pika. The user posted a picture emphasizing Ash's physical appearance throughout the series and how it has remained unchanged.
The text is in Japanese, but the chart includes every iteration of Ash from the Indigo League series to the upcoming series. As can be seen above, Ash Ketchum has maintained his general stature, and his facial appearance has never strayed too far from his first iteration. His pupils are a bit larger, though that is the only major difference. His Sun and Moon look is by far the biggest departure from his original design.
For the most part, Ash Ketchum has generally kept the same style of clothing throughout the series. Donning a red and white cap with varying emphasis, Ash has often taken to wearing jeans or jean shorts and a short-sleeved shirt or hoodie throughout the Pokemon anime series.
Overall, this is fairly inconsequential, but it is somewhat noteworthy that Ash Ketchum has largely had the same exact appearance over the past two decades that audiences have been familiar with him. It says that they have settled with an innocent-looking iteration that reflects someone who has yet to grow and learn through the experience of Pokemon training.
However, Ash Ketchum made perhaps his biggest stride recently when Ash won his first Pokemon League in the Pokemon Sun and Moon anime series. While Ash may not have matured physically yet, he has certainly grown more internally, which is part of what the series is all about.