OP. I work with teenagers and adults, so am very used to reading and listening to all groups.
The reason why some people find you teenagerish isn't slang and text speak. The way (in writing, so it could be quick typing not "you") you express yourself does sound quite " young". It's not text speak: you make your life sound very simple, not like you've considered other options or are in denial they exist. This tends to make your posts sound young/naive: there's no "we considered xyz might be a problem but were pleasantly surprised..." - any downsides are absent. Even very happy people consider and anticipate problems (and solve, prevent or live with them to be happy). Thus, some of your posts sound quite young.
Also your posts confuse discussion/debate with others (whom you identify as "feminists" though not every poster has said this) 'attacking' you. Again this can make your posts (not you) sound young because it is something teens struggle with more than adults (who can discuss differences without feeling threatened). Perhaps this dislike of discussion is just fast typing, perhaps it is fundamental to why you enjoy your DH taking the lead.
Finally, your points are often emotional (not practical so much) and -again could just be the posts/writing - the emotions seem quite simple and young. One sided with no self doubt. Possibly as they are forum posts.
Some posts you read and don't seem to understand the more complex subtle points made. Again, skim reading on a phone does this, but it might lead some to make assumptions.
The problem is, this might lead people to think YOU haven't thought things through or are innocent. So you're explaining and frustrated people don't believe you and act like you're young.
It isn't the text speak or slang, it's the tone of the posts. E.g. you grew up in a pub and don't like slang ... But in your 1st reaction to it you sounded all innocent and shocked! Making you sound young/naive. Actually, I think the innocent to slang, ladylike reaction to swearing is a deliberate choice on your part ... You want to be a traditional lady... Which is fine. But ladylike and prim is slightly different from "girly" (also fine) - it seemed you didn't realise how young you sounded in that post.