"Young at Heart" is the forty-second episode of American Dragon: Jake Long.
Jake's father, Jonathan Long, embarrassingly drops him off at school during a rainy day. Jake begins to feel that being a 14-year-old has no good effects on his life, wishing that if he were an adult he could drive a car and handle his own things.
Just then, Fu Dog contacts Jake to get to the Magus Bazaar to investigate a reported wild magical creature terrorizing the place. The trio arrive and witness the creature. Jake sends a photo to Fu Dog and Lao Shi, and warns them that the creature is an Avemetrus, who is as old as time itself. Fu Dog fails to warn Jake, Trixie, and Spud to stay away from its tentacles as it will suck their youth. Jake attempted to fight the creature, but the Avemetrus grabs the three of them. Jake then manages to escape while Trixie and Spud's youths are unknowingly sucked by the creature. Luckily, Jake manages to scare away the creature and it flees.
Jake returns home, heavily tired, and goes straight to bed. The next day, Haley walks into Jake's room, notices an unfamiliar person in Jake's bed and attacks him. However, she is shocked to see that it is actually a 21-year-old Jake. Jake tries to escape without being seen by his parents. Haley helps her brother to make his way up to the chimney, but Jake struggles to climb up it. His father then creates a fire and burns Jake, shooting him straight out of the chimney. After crashing, Jake dresses in other people's clothes and stumbles upon a now-elderly Trixie and Spud.
The three of them quickly get to Lao Shi's workshop, where Fu Dog explains that their youth has been stolen by the Avemetrus. When Jake questions how to revert them back to their original age, Fu Dog tells them that the Avemetrus has to be vanquished. While they investigate the Avemetrus' whereabouts, Trixie and Spud are forced to stay at a magical retirement home.
However, Jake, realizing that he is mature enough to do anything as an adult, begins driving lessons and performs horribly. However, he managed to convince his driving teacher to give him his license, in fear of ever being stuck teaching a horrible driver, just as long as Jake never goes near the driving teacher's neighborhood. Jake continues to pursue things as an adult, such as buying a car, watching PG-13 movies, and even purchasing his own luxury suite by just using a credit card, even though he has to pay it all back. Trixie begins to feel that Jake isn't focusing on hunting down the Avemetrus, as he is too caught up in the adult lifestyle and doesn't even care about the struggle that Trixie and Spud are going through. Jake feels sorry for them, so to make it up to them, Jake invites them to his luxury suite for a big party before they return to their normal age.
Hours later, Jake's suite is ready for the party. However, before it starts, Fu Dog tries to contact him, but Jake ignores his call. Unbeknownst to him, Lao Shi encounters the Avemetrus and can't fight it alone. He then decides to fall back, but gets ambushed by the Avemetrus' tentacle.
Back at the suite, Trixie and Spud make it to the party. However, shortly afterwards, Jake is confronted by a policeman, who tells him that someone had reported that Jake's party was causing a public disturbance, and he is ordered to shut down the party or be arrested. Jake realizes he can't escape, as he is not a kid anymore, and must convince the police to stop. Unable to stop the party, the policeman is about to bring Jake to the station when suddenly, Trixie breaks her hips after trying to "dance her booty" one more time. Things get worse when the Gnome that is after Spud shoots soda throughout the suite. Jake is overwhelmed by the responsibilities he has to face, and flees the scene.
Outside, Jake has a hard time realizing that adult life isn't what he thought it would be. He then receives an urgent message from Fu Dog to come to the shop immediately. Jake discovers that Lao Shi's youth has been stolen and he is growing very old. Fu Dog then tells Jake that if he doesn't vanquish the Avemetrus soon enough, Lao Shi will continue to age until he dies.
Back home, Jake calls for Haley's help to lure the Avemetrus for her youth. When the Avemetrus arrives, the two engaged the creature. Haley gets grabbed by one of its tentacles, but before it can steal her youth, Haley bites its tentacle. They are both thrown into a kid-stuff warehouse. Jake then devises a plan for the Avemetrus to steal its own youth. They harasse the Avemetrus with toys and the noise of arcades. Jake and Haley then land on its back. The Avemetrus tries to fend them off with its tentacles, but this results in it sucking out its own youth. It then goes out of control until the Avemetrus is finally defeated, and explodes. Jake, Trixie, Spud, and Lao Shi's ages are restored.
The next day, Jake catches up to Trixie and Spud, in bad condition after being forced to do hard work just to pay off his credit card debt. Trixie and Spud are proud of Jake for realizing that being a 14-year-old has its advantages. Brad is then seen driving past them, causing the water in the street to splash at them. While enjoying adult life, Brad kicks the gas pedal into full speed with his new car and crashes.
- The title is a play on the Frank Sinatra song "Young At Heart".
- This episode is similar to the Ben 10 episode "Don't Drink the Water".