This headcanon is unnecessary. Many women in their 20s can act like teens. I understand Faust wants to justify it but it really isn’t needed. Just look at Spongebob, who is literally a manchild who is more probably more relatable to kids than adults for the most part.

This probably doesn’t stop people from saying “they are totally in their late 20’s” to make themselves feel better about their lack of success in life.

It was a big part of MLP’s success. They acted like teenagers but were technically adults so they could be whatever was needed. The brony community existed because of fanfiction and the reason it thrived was because MLP FIM was believable enough yet still vague enough to fit into whatever was desired. You have MLP stories of all kinds from nonsensical to extremely thought out, from senseless porn to deep thoughtful tales, from cutie slice of life to dark stories of despair. It really succeeded in speaking to people’s imagination by setting a few rules but leaving a lot open to interpretation. It also was of high enough quality to really engage people and make them care enough to put serious effort into their works.
 
The way Lauren Faust writes characters also helped. She starts with a simple core or stereotype such as fashionista, or hyperactive candy pony and then fleshes them out from there. That means though that every character has a core that everything else is build around. That helps makes the characters more flexible because as long as people keep that core they can really bend the characters to fit their own stories without them seeming too OOC.
 
Proof of that is in the two arguably most influential stories of Past Sins and Fallout-Equestria. One stays very faithful to the show and keeps them all very in character with a very similar tone and morals. The other almost completely abandons the show and keeps it’s core messages and world in mind but nothing else. They really are on opposite extremes yet were both successful. Equestria could be a childlike world of wonder or a unforgiving landscape of greed.

hmmm