Bart Simpson has been 10 for over 30 years. Ash Ketchum has been 10 since 1997. Even Peppa Pig should be 21 by this stage! Children's TV characters rarely grow with their audience. In most cases, it makes sense for kids content producers to stick with their target market. Adjusting a character's age means you have to adjust their behaviour, appearance and sometimes their surroundings. This can be a time consuming and costly job that might not actually benefit a show's audience.
There are of course exceptions in the form of live action shows. Sabrina The Teenage Witch saw the title character go from 16 year-old highschool student to college English major to professional journalist. While moves like these are necessary in order to keep pace with the actual aging of the actors, they often find it difficult to maintain their audience. A 12 year-old might be able to relate to (or look up to) the actions of 16 year-old Sabrina, but might find it more difficult to relate to a 22 year-old struggling with the problems of employment and real life. In Sabrina's case, the show saw dwindling viewership numbers (a 9 million loss since its heyday) and poor reviews from critics.
That's not to say that including aging in content doesn't work. From 2003 to 2007, That's So Raven followed the adventures of a teenager with psychic abilities dealing with highschool trials and tribulations. The show was one of Disney's highest rated programmes during its initial run. Rather than continue the show with Raven as a young adult, the show was halted after 4 seasons. In 2017, however, the show was brought back under the name Raven's House. Rather than primarily focus on the adult Raven, the show instead featured Raven as a divorced mother of 2 children, one of which inherits Raven's psychic abilities. This new direction for the show enabled it to maintain its target audience, while also giving it a nostalgic appeal to all those who had watched the original.
Live action shows aren't the only ones to explore the concept of their characters aging. In Nickelodeon's animated TV show, As Told By Ginger, the characters aged across seasons allowing the creators to focus on a variety of different adolescent themes. Over 3 series, the show watched Ginger and friends go from seventh grade all the way up to junior high. The continuity in As Told By Ginger gave the writers more opportunities to explore various teenage topics. One of the characters goes from having invasive orthodontic headgear in the first season to being considered one of the most attractive people in the school. The aging up of the characters enabled the writers to tackle more serious topics such as depression and drug addiction.
Similarly to That's So Raven, another Nick animated show, Rugrats, also tried a matured spin-off featuring the same characters. All Grown Up! featured the toddlers from the original show but during their teenage years. More recently, Paper Owl Films' animated show Pablo has made the transition from preschool show into the 5 to 8 bracket. Pablo follows the adventures of a young autistic boy, whose autistic traits are represented by imaginary animals. In the new season of Pablo, we'll see Pablo deal with school for the first time and the challenges he may face. Aside from making Pablo look a bit older, the creators have also tweaked the animation from crayon-like art to more clean cut pen drawings in keeping with the age-up. If all goes to plan, the production team are also saying that they would like to see Pablo all the way up to highschool age.
Growing with your audience isn't always necessary. Shows like The Simpsons and Spongebob Squarepants illustrate how allowing your characters to age isn't always an appropriate tactic. Aging up also has the potential to deter your audience. Kids might associate a show with being "babyish" if they recognise it from when they were younger. While aging up might allow new plotlines to develop, there is also the argument that you may as well create a new show. Aging up might work for Pablo, but that's because it has a niche audience. Aging up worked for Raven but only because the show was able to appeal to its intended age group through Raven's son. Additionally, Raven could appeal to young parents who grew up watching the original. Shows need to put some careful consideration into whether aging up is appropriate for their brand, particularly when it comes to kids. While there are rare cases of it working, making a new show could be less risky than aging up.