I don't care if a little kid does it cause they're still real young. I'm talking grown men, some in their 20s.
I know one guy who was in my class who watched a episode of MLP and completely "ponified" his computer and constantly got on my nerves about it just to make me mad, he told me that was the reason as well.
Keep in mind that I don't hate bronies or anything, one of my real life closest friend is a brony. He has a toy or 2 and his computer has pony stuff but he still keeps to himself outside his bedroom.
even i find it a tad strange when i see someone completely transform a room with pony-related stuff. moderation and balance is key in all things. some adults remain kids, either by personality, teachings, or what have you. it's generally related to the upbringing and events they experienced over time that turn give you the later result. i'm not saying that's an excuse for behavior, however even adults (elders even!) act as such regardless of what they are into.
the kid you mention just sounds like an ass. probably was an ass before, and continues to be one after. there is no reason to act as such.
personally, i never imagine myself actively chatting with friends and family over this or displaying pony stuff in my room. the internet is the only place i've really discussed it, and i don't mind keeping it there in its place.
also, i hold nothing against you or any others in the topic regardless of their viewpoints (accurate or not). i simply try to provide accurate information based on what i know, as unbiasedly neutral as possible