Pokémon's Ash Ketchum has been 10 years old for 25 years -- but that doesn't mean he hasn't grown up a little in that time. From his start as an oversleeping kid who just barely managed to get his starter Pokémon, to the Alola Champion and current research assistant to Professor Cerise, Ash has been changing and maturing along his journey, growing into a better person.
Back in the days of the Indigo League, Ash's personality was almost unrecognizable compared to his current-day counterpart, aside from a few key traits. The first and most obvious of these key traits is his tendency to rush into things headlong without thinking, such as when he attempted to catch a Pidgey by throwing rocks at it in the first episode.
Another arguably more important aspect of Ash's character is his willingness for self-sacrifice -- from protecting Pikachu in Episode 1 to basically dying multiple times in the movies, Ash has no problem putting his life on the line if it means doing what's right. He also cares deeply about his Pokémon, and respects their decisions -- whether it's evolution or love, Ash won't force any of his Pokémon to do anything they don't want to.
But Ash's love of Pokémon doesn't necessarily translate into maturity -- particularly since he held very little love for his fellow man at the beginning of the series. As the show began airing in the late '90s, Ash was trapped by some of the thinking of the day and often mocked Misty for her femininity -- despite the fact that she was usually a far more capable trainer than he. Likewise, the Ash of the original series struggled greatly with pride and overconfidence, boasting about his accomplishments while putting very little work towards those accomplishments.
He was quite lazy, taking every shortcut he could think of to bypass the hard work needed to earn his badges -- of his first five, only one was earned by defeating the gym leader in a fair match. While his kindness earned his Pokémon's loyalty, it was their strength and not his skill as a trainer that earned him a spot in the Top 16 of the Indigo League.
Luckily for Ash, he had plenty of opportunities to learn. Traveling with Misty through two regions allowed Ash to see how strong girls could be. When he met May, he regarded her as an equal and supported her goals, setting a precedent for the rest of his journey. Likewise, Charizard taught Ash that kindness alone wasn't enough when it came to training Pokémon -- he needed to earn their respect, give them space when needed and work with them to develop their own battle style. He stopped taking shortcuts in the Orange Islands, and throughout Johto and Hoenn, he started working on training with his individual Pokémon to not only become stronger but to deepen the relationship between himself and his Pokémon -- a trait that has continued to this day.
Ash also started to take notice of others more as he left Kanto. Where before he had been self-centered and boastful, he now discovered joy in helping others achieve their dreams, such as encouraging Clemont's inventions or helping Mallow find Yellow Nectar to make her restaurant's legendary dish. As a result of growing to care for his human companions more, he grew not only to be able to accept defeat but even sincerely congratulate his friends when they bested him, such as with Iris in Season 14, Episode 42, "Club Battle Finale: A Hero’s Outcome!" In the same sense, Ash also became more independent as the show went on -- from relying on Brock to take care of his needs to becoming a trainer who could travel comfortably alone. He was even able to take the lessons he had learned through his travels and use them to help others, teaching May the basics in Hoenn and confidently mentoring Dawn in Sinnoh.
Perhaps the most important way Ash has grown, though, is with respect to his rivalries. When the series started, Ash's rivalry with Gary could be conservatively called "toxic," with Gary constantly berating and besting Ash in every possible way, perhaps leading the kindhearted young man to emulate Gary's boastfulness and arrogance. That all changed when Ash finally defeated Gary. He didn't do this through sheer power, but through quick thinking and use of the environment -- exactly the kind of unorthodox battle style Ash is now known for. The defeat allowed Gary to see Ash as an equal and Ash to put their rivalry behind them. Both have become better because of it.
Nowhere is that foundation more important than with regards to Ash's rivalry with Paul in the Sinnoh Region, which helped to solidify Ash's training style and motivations. Paul believed that only strong Pokémon were worthy, releasing any that weren't up to his standards regardless of the Pokémon's feelings. Ash spent much of the Diamond and Pearl series seeking to prove Paul and his harsh, callous methods wrong by showing that any Pokémon can become strong through training, hard work and kindness. While Paul's methods showed instant results, the bond of respect and love between Ash and his Pokémon allowed them to finally defeat Paul when it mattered.
While Ash may remain 10 forever, he's certainly grown a lot throughout the series, and he's not done yet either. He still rushes into things with enthusiasm befitting a kid his age, and has yet to learn the meaning of "tact." Still, Ash has mellowed out significantly since his first appearance, become more accepting and compassionate towards others, even humbling himself as he seeks to become a better trainer worthy of the title "Pokémon Master."