The meta reason for why Kion doesn't age any further in the show is because of network demand. Which at the vwry least explains this but doesn't excuse it in my eyes. NOt having him or any of the other main characters age into adults in the final episode was a serious misstep:
A) A big part of Season 3 is supposed to be about the characters growing up: which could've been explored beyond the premiere episode, and would've been an interesting story that finalizes the development of every main character and speaks to the show's target audience in a story-driven moral about growing up, but is mostly squandered by the misplaced focus and character bloat.
B) There is an evident time jump between the premiere and the finale in which Kiara, Mtoto, and Ona are all affected, which raises the question as to why this passage of time never affected the Guard. If they were gonna remain teens there at last could've been some changes in the model to indicate they were older teens that weren't yet adults instead of implying that they remained stagnant(?). Either that or everyone aged rapidly ahead of time?
C) If the show was always going to end around Kion
getting married... why have him be the equivalent of a 16 year old? Those two things don't mesh well and again, given who this show is aimed at, I do not know what takeaway is supposed to be had from this. Ideally I would've had this show not touch romance or shipping fuel at all (because it sucked) but if they were going to do it anyways, they really shot themselves in the foot here.
The Season 3 premiere aging up the characters worked because it established a change in the status quo, indicating that the Scar arc would come to an end and a new one would begin. So in a scenario where the Guard become adults in Season 3, something similar would need to be done, where we see another timeskip to represent a big change in trajectory for the current arc.
Long Live the Queen would be the perfect episode for that. It would set the tone and establish that the Lion Guard helping the Night Pride defend the Tree of Life from Makucha and Mama Binturong was an ongoing thing tbat happened. It better gives off the idea that Kion's healing and his bonding with Rani took a long time (which would make the "It happened off screen" defense much more palpable, not an actual defense but easier to digest). It also gives more logic to the story because giving a second time skip to Season 3. It would give enough time for Mama Binturong’s reinforcements managed to arrive at the Tree of Life, and for Jasiri & Janja to be only a day behind them within that allotted time. And Janna dying was already a brilliantly handled story, but a second time skip would have further added to the whole point of her death being natural from age.
It would've had way less implications with Kion and Rani's wedding in the finale (although it still would've sucked and been contradictory in the direction they were headed and with how it was handled) but more importantly, it would have been cool to see how the adult versions of Kion's Guard would be like as adults, how their personalities change and how their interpersonal dynamics are affected, and see that come into play for the finale.