Definitely not a kid only thing. Certainly some elitism but I think mostly annoying to deal with a single green disabling all the iMessage features for the group, hence greens frequently get left out or communicated with separately
https://www.macrumors.com/2018/03/09/quick-takes-mar-9/
Part of this article speaks to the iMessage:
"Commentary: A good example of the strength of Apple's ecosystem is iMessage and its
coveted blue message bubbles, which have essentially become a status symbol. As silly as it may sound, there are a
countless number of tweets that mock green bubbles, which is the color Apple uses to display standard text messages."
So it's a widely known phenomenon.