Looking for a stack exchange where I can ask a question describing the context of a story/character idea, and request ideas on how to fill in certain holes or develop it so that it fits certain events and criteria (for example, questions like "what kind of backstory would allow for this character to have a knowledge of [thing] at a young age?"). However this feels too specific to post in the general writing stack exchange, and more akin to the types of questions on the worldbuilding forum here. Does a stack exchange site like this exist (and is it actively used much), or is there already a more general fiction writing stack exchange site that will still accept these sorts of questions?
There is https://writing.stackexchange.com/ but they focus on the craft of writing, and like us they do not support questions about plot/characters. Please do not post there asking for help with your plots or characters.
Questions about plot and characters are very subjective, with many equally valid answers, two things that are fundamentally against the Stack Exchange ethos. Worldbuilding is already pushing the boundaries of the Stack Exchange format. A site where people offered their opinions and generated ideas about plots and characters would be far over the line.
NOT YET, NOT HERE
The kind of forum you're looking for would not be possible at Stack Exchange. If you look at your example question, what kind of backstory would allow for this character to have a knowledge of [thing] at a young age, you'll see that you have essentially asked for everyone's opinion.
SE's basic model is to take a focused query and give it a definitive answer. Every forum except for WB.SE focuses on real world issues; we're the only forum that delves into fantasy and fiction as realia.
You're looking for a brainstorming forum! This kind of question might be suitable for one of the Reddit worldbuilding groups, if you're just looking to bounce ideas around. If you're coming at this from a more serious writing perspective, Mythic Scribes might be a good choice.
It's still in a proposal stage, but the idea to create a forum similar to WB.SE but without the strict SE constraints is still alive at Codidact.
I don't know any website
Even looking the big resources list, I doubt any of those forums/reddits will match what you're looking for. As Sphennings told, it's more prone to discussion, so a forum of passionate writers is more likely to help than a stricty Q&A site.
And as a pure worldbuilding question, it's hard, very hard to write these kind of questions. It's too much focused on a character to be considered as how a society works, or what such society finds important on someone's background.
At the same time...
There are some places you can easily query the question you're bringing up :
- People around you :). It's easily the easiest and easier solution. Bring a cookie bag and get the brainstom rain up. If you're shy, bring subvertly the topic by talking about other works where the same knowledge is needed of a character. Just be sure to distinguish what you think and what the others think, it gets muddy quite easily.
- For real-world activities which require specific knowledge, there are many videos and website talking about them, be it offical job vocation guides or amateur and expert talks. There's always interesting things there.
And most importantly, these things depends on the individuals. Like I told above, it's very focused on the person itself, and more often than not every trait they have is both a boon and a malediction in and of itself. Let's take an example and you want to know what would make a good classic fantasy adventurer :
- They were a cartograph : Drawing maps is ultra-easy for them, but they're expert with a pen, not a sword.
- They were a war-mercenary : Drawing blood is ultra-easy for them, but they were not the one making the map, and they'll get lost if not careful.
I'm not even pushing into the personality traits each of them have (e.g.: an abrupt and determined soldier, or an elegant and peaceful cartograph). The point is : As long as you know the inner strength and weaknesses of your characters, you'll find in them what you want to give them. And if that's still too difficult, remember that a lucky opportunity can always happen (I don't know how to read maps, but I just happen to have listened at the tavern one guy who knew the path!).