There are many reasons why mentally mature people also find an interest in content designed for immature minds. The first of which that I can list, in regards to bronies and pegasisters being fans of My Little Pony, especially Generation 4, is:

1. That kids shows are chock full of moral lessons that everyone can benefit from.
2. MLP may have more compelling drama plots per episode compared to ‘adult’ soap operas.
3. The desire for a fantastical escape from a world that’s becoming increasingly negative.
4. Psychologically I suspect the characters or settings were very relatable, perhaps in terms of innate desire to:

• Fit in (Twilight)
• Be happier and less depressed (Pinkie)
• Be useful to society/have a loving family (Applejack)
• Be peaceful/coexist with animals (Fluttershy)
• Be popular/cool/more confident (Rainbow)
• Have self-pride/be helpful to friends (Rarity)

5. The fantastical idea that conflicts can be solved with love, not war.
6. Many animations are often designed with references or jokes that only adults would know, and keep them entertained. For example, how many children would recognise Dashie’s morning wing-boner as anything of note, or that one episode was a parody of Murder on the Orient Express?

Either way, something relatively unique and wonderful happened with MLP:FiM.