Why ONLY the Sm4sh community? Could it be because of younger players playing the game?
Honestly, I see it as a symptom of many different problems not necessarily related to Smash combined with an essential core aspect of Smash 4 that enables this toxic complaining culture within the Sm4sh community.
Firstly, I think age does have some part, but this age issue is present in other smash communities as well. Smash 4 community is young, but so are other Smash communities atm as new players come and go. And of course, this issue will most likely be exacerbated with SSBU coming.
Another problem is that social media, as well as sites like reddit, unfortunately encourage mob mentality and negative drama to perpetuate. I'm not really sure what the best solution is for this because the issue of mob mentality is a problem that extends way beyond Smash. Perhaps encouraging people to more actively use and post on Smashboards by recruiting and incentivizing Smash content creators of all kinds to post on Smashboards might be a solution because Discord, imo, creates too much fragmentation within the community, but I digress.
Finally, the essential aspect of Smash 4 that really enables toxic complaint culture to persist is
patches. And by saying this, I am not implying that there is something inherently more immature or bad about Smash 4 players. But the fact that patches and patch culture existed in Smash 4 for a significant amount of the game's lifetime (and most likely will going forward in SSBU as well) inevitably lead to many people complaining and being very vocal about their salty opinions. And this is because it is just soooooooooo much easier to complain about things you don't like about the game, rather than learning to counter it properly or respecting it.
This becomes especially evident in Smash 4 and its community when you realize that most patches in Smash 4 directly addressed and validated people's complaints and salty outcries, such as Diddy nerfs, Sheik nerfs, Rosa nerfs, Little Mac nerfs, Luigi nerfs, etc etc. And once again, I want to
stress that this is
not because Smash 4 players are inherently more salty or immature compared to other Smash players;
everybody can get salty and complain about anything within a game for whatever reason,
even top players. For example, Leffen, a top Melee player and a high tier DBFZ player, routinely complains about certain top tier characters in DBFZ in twitter and have made numerous posts about begging for the developers to nerf them. Even though Leffen regularly competes in a game with no patches, and thus one would expect him to not be such a big complainer and know the value of overcoming difficulties within a game, regardless he regularly complains in another fighting game community.
Of course, his endless salt against Jigglypuff is quite infamous as well. But the most important fact about Leffen's complaint against Jigglypuff in Melee is that, at the end of the day, you have to adapt or you will lose. No matter how much you complain about a certain character being dumb, broken, OP, cheesy, etc, there are no patches in Melee (and other Smash games beside SSB4) to validate your scrub opinion and save you from overcoming difficulties within the game.
You have to "git gud", essentially, or you will never stop losing. In conclusion, patches are generally not good for the overall health and longevity of any fighting game because it quickly promotes scrub behavior from the playerbase and eventually leads to degenerate mob mentality, no matter how mature the players may be. And the Smash 4 community really is a clear example of this imo, what with the recent fiasco at Evo 2018 GF and a toxic, mean-spirited twitter response from someone called Parappa (and Sinji's support of said tweet) towards Lima's incredibly vulnerable and relatable twitlonger post (twitter links below):
https://twitter.com/LimaBayo/status/1032660469151145984
https://twitter.com/Parappa_SSB/status/1033019113386459138
https://twitter.com/Sinji93/status/1033128791047118848
Of course, not all patches are bad. I love patches that actively buff lower tier characters or characters that are not meta-relevant, but this is a tall order to ask from the developers of any fighting game because they are pressured by the playerbase to respond to not just buff requests but also nerf requests, and that's preciesly when **** starts going down.