* The genre of your story. If you're writing a children's book, your protagonist will likely be a child. If you're writing a young adult novel, your protagonist will likely be a teenager. And if you're writing an adult novel, your protagonist will likely be an adult.
* The setting of your story. If your story is set in a time when people typically had shorter lifespans, your characters will likely be younger than if your story is set in a time when people typically had longer lifespans.
* The plot of your story. The age of your character will also depend on the role they play in the story. If your character is the main protagonist, they will likely be older than if they are a supporting character.
Once you've considered these factors, you can start to narrow down the age range for your character. If you're still not sure, you can always do some research on the average age of people in the time period and setting of your story.
Here are some common age ranges for characters in different genres:
* Children's books: 3-12 years old
* Young adult novels: 13-19 years old
* Adult novels: 20 years old and up
Of course, these are just general guidelines. There are always exceptions to the rule. For example, you could write a story about a 20-year-old who is still in high school, or a 50-year-old who is still raising young children. Ultimately, the best age for your character is the age that makes the most sense for the story you're telling.