President Andrew Shepherd: "She didn't say anything about me?"
For the most part, as someone grows older they become more mature and grounded in reality. They pass by the immaturity of their younger days and become an adult.
But there is a phenomenon where in a specific situation or when around certain individuals a person may turn from their confident adult personality and revert into a nervous teenage (or younger) incarnation. It may be because of interacting with an old friend and they reminisce about all their old activities they did together. Or maybe they have found themselves re-enacting a situation from their past they thought they had grown past, such as breaking up with a long-time partner and finding themselves back in the dating pool.
A common gag with this involves a Parental Substitute
where a "more mature" friend may have to be the voice of reason and they bicker with things like "You don't understand me!"
- Briefly poked at in The American President, where divorcee President Shepard found himself attracted to a lobbyist and sought to find some way to pursue a relationship with her without causing too much attention. His attempts ended up making him sound like a middle-school kid wondering if a girl liked him.
- Addressed in full in Home Improvement where a college friend of Tim's was in town, who was not liked at all by Jill. Tim realized that all their time together is spend reminiscing about their "glory days" and trying to re-enact them. Since Tim had a family now he couldn't go off to a bar like his friend wanted to do and the overall message was that true friends grow up with you, which his college buddy did not.
- Made into a major Running Gag in an episode of How I Met Your Mother. Robin's first boyfriend came to visit and despite still being in the same band with no music label backing them and working at a water park to support himself, Robin reverted to her starstruck teenage personality. This lead to a discussion among the group about others who do the same thing, such as Ted having a playful push-fight with his high school buddy Punchy and Lily speaking in heavy street-slang when she meets up with her friend Michelle.