I think everyone is agreed on the fact that it has nothing to do with age or content per se. That means writing a five-year-old character isn't immature writing, neither is having that character speak in baby talk or have another, grown up, character use baby talk with them. If it's authentic for the character it's good writing, not immature.
After that, things get a little muddy as opinion creeps in.
Some people believe that immaturity means not sticking to all "the rules". I do not adhere to this theory.
Reading that someone has their eyes glued to a screen doesn't pull me out of the action to wonder how he gets them unstuck (and trust me, as someone with asd that's saying something). Neither does it when, having unglued his eyes, he allows them to rove around a room and settle on the object of his desire.
I have no problem with author asides, such as "He was so deep in thought when he left the hall, he didn't see the dark shadow that slipped from behind the bush and watched him walk away." We're in his point of view so if he didn't see the shadow, neither did we. But the author was setting an atmosphere, as well as clueing us in to the fact that something is going on. This colours how we view what happens in the story from then on. Personally, I like this. Technically it's head hopping and "not allowed". I have to wonder who made the rules and who is allowing and not allowing what I write?
To me, the whole question of whether writing is mature or immature centers around syntax - the way the writing sounds - out loud and in your head, and the way it makes you feel, think or perceive your world.
Some examples of immature writing, that I notice when I review and have struggled with myself are
- Using names in dialogue. Think about it. When you're talking to someone, how often do you actually use their names? You can go days, weeks, months, maybe even years without saying someone's name, unless you're calling them or emphasizing something? "For goodness sake, Cher, what are you doing now?"
- Overusing "that". I always do a search for "that" when I've finished writing and try to take out as many as I can. Most sentences make perfect sense without it and it stilts the writing and interferes with the flow.
- Not using a name enough outside dialogue. Especially in m/m it gets extremely confusing when all you're saying is "He did this to him, while he was doing something else, but he didn't like was he was doing and was cross with him for not thinking about what he wanted before he did it."
- Using "he" and "she" at the start of sentences. It's fine as long as it's only once in a while. For the same reason as above, it's best to use names if you can, so readers know off the bat who they're focussing on.
- Starting a sentence with "Also" or "Then." "And" and "But" come into this category, but there are times when it's okay, mostly for emphasis. This is because they're "joining words" so should be in the middle not at the front.
- Starting a sentence with "Now". It can be okay in dialogue - remember just about any rule can be broken in dialogue - but not otherwise, except in very rare circumstances.
- Writing lists. "He looked over and saw her, and smiled at her, and walked over, and said hello, and smiled again, and...." As a rule of thumb don't use "and" more than once in a sentence.
- Not using contractions. For me, nothing spoils the flow of a piece of writing as much as under use of contractions.
- Something I call "Opening loops without closing them properly." For example "Before I closed the door...." What? What happened before you closed the door. "Although it was cold...." What about it being cold? This leads me on to another point
- Being attentive and consistent in language used. "Although it was cold he was shivering." Um no. "Although" means "in spite of" or "even though". Therefore the sentence has to resolve with something unexpected. "Although it was cold, he was sweating." "Even though it was cold, he was wearing shorts." Similarly "Not only did he...but..." More on that in the next part but for this point the two words have to have a connection. "Not only did he make her cry, but he had a sandwich," makes no sense because having a sandwich had absolutely nothing to do with making someone cry. "Not only did he make her cry, but he ate her sandwich," is much better because there is a logical connection.
- Correctly using paired words. For example, if you use "not only" you have to use "but". "Not only did he....but he...." So you don't say "Not only did he make her cry, then he went for a walk." It sounds pretty obvious but I've seen similar, although perhaps not so blatant, examples.
- Another example is "neither...nor". "The answer was neither black nor white". This often gets confused with "Either...or...." "Either the answer is black or white." "Neither...nor..." suggests something is neither one thing nor another. "Either...or..." suggests something is either one thing or the other. They are qualitatively different. And, very importantly, they can never be mixed. "It was neither right or wrong." "It was either right nor wrong." Both are very wrong.
- Repeated words. I fall foul of this all the time. Basically, it refers either to words that crop up too much during the work as a whole, or too many times in one sentence/paragraph. For me, my buzzwords are look, eyes, smile, beautiful and really. As with "That" I do a search through the documents for the words and eliminate as many as possible,
- Passive voice is a big one and tricky to spot. I've seen it called filtering and I like that because it is essentially an action or event being filtered through something else and it almost always makes the piece weaker. For example. "A knock came at the door." or "He heard a knock on the door." It's so much stronger and cleaner to say "There was a knock at the door." Or "He reached out to touch her hand." "He touched her hand." "He felt that it would be a good idea. ""It would be a good idea."
- Using unecessary words. "He nodded his head." What else could he nod? "He nodded" will do just fine. "He sat down." He isn't going to sit up, at least not with the same meaning. "He shrugged his shoulders." Unless he shrugged his eyebrows don't bother to specify because everyone knows what you mean, although I do wonder about this when readers are unable to understand that eyes being glued to a screen don't require paint thinner to get them off.
I'm sure there must be many, many more so please feel free to clue me in, in the comments below, in fact I would love you to.