Sentence length affects your reader's experience. Texts with lots of long sentences will exhaust your readers. On the other hand, texts with lots of short sentences will create a choppy, disjointed reading experience.
Your chosen genre should drive your average sentence length target. In the chart below, you can see that writers in different genres use very different average sentence lengths, depending on whether their primary aim is to inform or entertain.
For instance, the chart shows that Young Adult authors tend to use the shortest sentence lengths. This makes sense as their writing needs to be easily accessible to younger readers. At the other extreme, Popular Science writers such as Richard Dawkins and Stephen Hawkins average 20 words per sentence. They're explaining more complex topics to a more mature audience.
If you want to write a page-turner for general audiences without specialized academic knowledge, then stick to the shorter sentence lengths favored by Thriller and Romance writers.
How Sentence Length Affects Readability
There's a correlation between readability and sentence length. As average sentence lengths increase, the grade level (number of years of education) required to read the text also increases.
Most published fiction has an average grade level of 4 to 6. That means it's accessible to ages 9 to 12. If you want your writing to be accessible to a mass market audience, you should write at those grade levels. Many beginner fiction writers are amazed by how low their target grade level should be
If you need to increase your sentence length, try using coordinating conjunctions like "and" or "but" to join together sentences with similar ideas.
The key to reducing long sentences, on the other hand, is to focus on the "meat and potato" words of the sentence. These are the words that give the sentence meaning: the subject, verb, and object.If you're trying to reduce the length of a sentence, start by writing out the meat and potato words. Then ask yourself what other information in the sentence really needs to be there.
The answer? Only add things that add valuable information to the sentence. If you're not sure if they're needed then they probably aren't!