Adventure time is a creation myth and a coming of age story. There are likely a million video essays on youtube about this. Overall the story follows 14 year old Finn and his magic dog brother Jake as they do hero work in the land go Ooo. Over the course of the series Finn matures in his relationships, learning to appreciate his friends even when they don't fit his narrow expectations of who they should be. Take Princess Bubblegum. As the knight in shining armor, Finn expects to save the princess and get the girl. But it is quickly established that Bubblegum is way too old for Finn and sees him as a brother (literally Bubblegum is centuries old). Finn does not pass Go, he does not collect $200. By the end of the first arc Finn has learned that love takes many forms, so while he loves Bubblegum, his romantic attraction to her is shallow compared to the deep well of their friendship.

These kinds of revelations (some more involve; fear, death, expectations, loneliness, fatherhood etc.) dovetail nicely with the lore of the show. Finn is the reincarnated entity of creation, preservation, and heroism. Our whole cast of characters are in some way reincarnations of the universe. There are four elemental beings; ice, fire, slime, and candy. But there are also agents of the universe like Finn. Where Finn is preservation and heroism, the Lich is an agent of desecration and decay. We also have fundamental characters like Prismo who controls the flow of time, the Cosmic Owl who controls the cosmos, and Death who presides over...death.

This list is not exhaustive when it comes to the magic system and overall lore of Adventure Time but I'll finish with the characters that are central to the final conflict; Magic Man, Grob Gob Glob Grod, and Golb. Yes, I'm sorry, it's confusing, they're brothers. These three characters are Martians. Magic Man is disgraced and sent to Ooo for war crimes, Grob Gob Glob Grod is a four headed god character who presides over the rule of Mars with a statue of Abraham Lincoln, and Golb is the destroyer of worlds.

These characters have wild premises and wild deliveries of plot points. The overall tone of the show is lighthearted but sincere. The animation is zany but at times disturbing, as is the music. But overall it links back to a rich lore. All of these elements can attract young and old alike.

If you want to check out what any of these characters look like, or more about their backstory heres the wiki:
https://adventuretime.fandom.com/wiki/Category:Characters