The people posting on your forums just haven't learned yet that if you want to interact with other people it helps to follow the common unspoken rules. But this is their age and selfcenteredness, not their spelling skills at work. Plenty of older people who are self-centered start a forum post with "HELP please" in the subject, forcing anyone to open the post to see what the actual problem is... bad spelling? No, just not being able to do the mental work that other people have their own lives and so if you want their help you ought to make that as smooth a process as possible.
While I will concede that the kind of behaviour you describe is annoying and self-centered, and I do agree with the general spirit of your post (minus the get-off-my-lawn attitude), I really don't think it's an age-related issue. You don't know how old the people you describe are, because the only information you have is their username and the content/context of their post. Sure, sometimes this makes it fairly easy to infer all sorts of information about the poster, but it's also easy to fall into the trap of taking those inferences too far.
Assumption: Most people "inhabiting" the internet are young (however you choose to define "young").
Observation: A lot of immature behaviour online.
Conclusion: Young people don't have the social finesse/foresight/whatever needed to effectively communicate.
Sounds promising, but let me provide a counter-example as food for thought: the Internet as such is largely a "home" to the generations that are in their late twenties and below. We have seen a completely different set of 'unspoken rules' for social interaction pop up in the virtual world than the existing ones governing meatspace. In my experience, and like you point out yourself, a lot of people behaving stupidly on the internet are in fact older people (again, I'll leave the definition of "old" very loose) who just don't "get" these new rules. Sure, the majority of inflammatory Youtube comments (still the #1 font of online stupidity, even though with the comment rating system it's gotten a lot better now) are probably written by 14-ish year-olds who don't know better, but now (age 21) I see a greater proportion of people from my mother's generation using caps and exclamation marks in inappropriate quantities than I do from my own.
In short, it's not just the gen. Z, post-post-baby-boomer generation gappers that are stupid; it's everyone.