Go Young, West Man! is the fifth episode of Season 5.
Having learned that Catman wasted his entire childhood in the TV business, Timmy Turner wishes that Catman could have one last adventure as a kid. Things are going fine at first, until the pair of boys are spotted by Dimmsdale's local truancy officer, Shallowgrave. Timmy must now outrun the wrath of Shallowgrave while also having fun with Catman.
Timmy and Adam West are playing croquet when Timmy asks Adam about his childhood. As it turns out, Adam was a child star as a kid, so he didn't have the same kind of childhood Timmy had. When Adam goes to grab something from inside his mansion, Timmy and his godparents feel sorry that Adam didn't get to have a normal childhood, and Timmy wishes for Adam to be a kid again so he can live out the childhood that he never got to have (while convincing Adam that he's just acting rather than have Adam discover that Timmy has fairy godparents).
Timmy's parents let kid-Adam stay with them, but when it's time to go to school, Adam (who was apparently homeschooled as a kid) suggests skipping school, which Timmy agrees to. Principal Waxelplax notices Timmy and Adam skipping school, and sends Truant Shallowgrave after them, advising him to bring the kids back alive (to which Shallowgrave charges extra money).
The first thing Timmy and Adam do is hang out at the arcade, which is deserted except for them, and then they hang out at an ice cream shop. Wanda suggests that Timmy and Adam head back to school before Shallowgrave catches up to them. After getting done with the ice cream, Adam suggests that they go and play pranks on people--their first victim being Timmy's teacher, Mr. Crocker (they decide on TP-ing Crocker's house). It's at Crocker's house that Shallowgrave catches up to Timmy and Adam--although Timmy manages to get rid of him after wishing that a clown on a unicycle would hit him (it was the only thing that Da Rules would allow for him to do, as the other stuff he tried wishing up were technically lethal).
After getting rid of Shallowgrave, Timmy and Adam end up hiding out in Timmy's treehouse in the backyard of the Turner home. Timmy points out that this would be a rather obvious and predictable place for them to hide, but Adam's convinced that it will work. However, Adam's proven wrong when Shallograve shows up outside with a bunch of police officers--Shallowgrave also claims that when they get Timmy and Adam out of the treehouse, they're going to send Adam to the pound, which gets Adam scared enough to hide under a rug in the treehouse. Timmy then decides to just suck it up and give himself up to Shallowgrave and the police (which ends up surprising Shallowgrave).
Adam sees Timmy do this, and Cosmo and Wanda end up poofing him back into an adult after deciding that something he said was the closest they could get to a wish that Timmy would make. Adam (now an adult again) comes out of the treehouse and claims that Timmy was with him the entire day, and as long as Timmy's parents were okay with it (which they admit that they were), Timmy didn't technically break any rules or laws, so Adam was "in loco parentis" (a legal term that means "in place of a parent" in Latin), which Adam explains is something that he learned as a child star when he was on a show called "Doogie Catkid, Junior Lawyer."
After this is explained, Timmy's off the hook--although Shallowgrave vows to get him in the future before flying off on a rocket. When Timmy points out that something Adam learned as a child star ultimately saved them, Adam realizes that his childhood as a child star wasn't such a bad thing after all. Everything's back to normal, and the episode ends with Adam hacking up a hairball on the screen.