In Digimon Last Evolution Kizuna we learn why the digi destined are always kids. Basically it is because of there endless potential according to a new article over at Comicbook.com.
“Digimon Adventure: Last Evolution Kizuna’s main conflict is a literal countdown to the end as Tai and Matt discover that Digimon partnerships reach a very prominent end when a child grows up. It’s revealed that Digimon pick a child to be their partner because they have boundless potential for growth. That potential converts to an energy for the Digimon.
As Digimon grow alongside their partners, children are the best option since they have that potential for growth that adults do not have. As a person grows older, that energy dwindles and soon that Digimon is nowhere near as strong as it used to be. Even more so the film reveals that when the chosen child ages into adulthood, their Digimon partner ceases to exist.”
Now marketing wise it is because you want to appeal to kids thus having kids as the main works. With that said it is nice to have an lore reason as well.

Article:https://comicbook.com/anime/news/digimon-why-digidestined-are-kids-explained-last-evolution-kizuna/
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