Young at Heart is an episode from season 2.
Plot
After Jonathan Long returns Jake back home, Fu Dog and Lao Shi sent Jake, Spud, and Trixie to Magus Bizzare. The 3 heroes see 3 trolls running from something. Jake takes a picture of the monster that attacked the trolls and Fu realizes that the monster is an Avimetrus. Jake breathes fire on the Avimetrus, but the creature grabs Spud and Trixie and starts feeding on their youth. Jake scares away the monster and releases his friends.
The next day Haley sees that Jake looks different and Haley realizes that Jake is an adult now. He looks at his reflection and sees he is an adult. Trixie and Spud are elderly when they arrive. Back at the shop, Fu warned them that they were supposed to stay away from the Avimetrus's tentacles. Jake tells them that the monster that attacked them is called the Avimetrus, and Fu says this creature has the ability to live forever by feeding on other peoples' youth. According to Fu Dog, to restore the victims to get their youth back, the Avimetrus must be vanquished.
Spud and Trixie and Jake went to visit old magical creatures at a retirement house, such as mermaids, strigois, gnomes, and unicorns. The 3 heroes later go to a party.
All of the sudden, the same Avimetrus one at Magus Bizzare, attacks Fu and Lao Shi. Jake arrives back at the shop and Fu has shows him that Lao Shi is very, very old after his youth was stolen by the Avimetrus. Fu Dog tells Jake he needs to hurry to defeat the Avimetrus, otherwise the old man is on his way to bye-bye (means that Lao Shi is about to die).
Jake needs Haley's help to rid of the Avimetrus. The Avimetrus grabs Haley and just as he was about to feed her youth, Haley bites the Avimetrus's tentacle. Haley and Jake crash into a toy shop and the Avimetrus approaches.
Haley and Jake have an idea: they plan to make the Avimetrus feed on its own youth. They distract the monster and they wrap it's tentacles around itself. The Avimetrus starts to die and it explodes. Everyone's youth is returned.