Doesn't matter the age. Old and young people can be extremely immature in regards to things like this. (See it on Twitch daily. Part of the reason why I lurk in people's streams for days before I follow them.) It is just common sense not to recreate certain things in a game like this. Nearly every post I've seen when this went viral has had a lot of laughter in the comments. People who do stuff like this just don't care and it's not restricted by ethnicity, age, gender or wealth level.
Even if they are kids... They know not to laugh at other things (i.e. gen*cide, hol*caust, etc.). But somehow 9/11 and racism are funny? I wasn't born when other horrible events happened, but I know not to laugh at them.
I feel a shift has happened recently and more people feel emboldened to show they don't care about the deaths/tragedies of others.