The Aging Process
When it comes to writing, one thing that should be transparent is the age or gender of the author. When a writer writes, the context of what is written should not show the age of the author or even the gender. If you didn't know anything about the author, you should be able to read the work without realizing if the author is male or female, or older or younger. Yes, some stories give away pieces of the author, as many incorporate things about themselves into their work. It is their work, after all.
Characters are also a part of the work and part of the author's imagination, and the part of themselves. Writers need to be careful when writing, so that their age does not influence the age of the characters.
I once read an author's work (who was in their early 20s), and the characters were supposed to be in their 30s or older, and they acted immature or childish, in a way that was not endearing. The book came across as immature. Writers need to know how old their characters are (or perceived to be) and sometimes that's something that grows as they write for them, but it also determines how they are shown in the story.
Names, dialogue (words, phrases), appearances, gestures, movements all determine and show the audience how old a character is (or at least an age range). Even the immortal characters should show a little wear on the centuries that they've lived, and not act as if they were born yesterday.
Research popular names for years that a person may have been born. Also, keep in mind that a person who is named Mildred, Arnold or Freda, may be a young character, but there is reason they have that name.
When I name my characters, it sometimes fits with the uniqueness of the story (I have a character named Sunshine Rainbow Savvy Macavoy, because she was born on a hippie commune) I've also have a personal thing (quirky to some) where I like names that start with E or M. Sometimes (even I admit it), I use a name in several different stories.
Surnames are also important, as they show culture, and they should match the culture of the character. The culture and how it is represented also shows the age of the characters.
A writer does not have to tell the audience that his/her character named Bill is 50 years old. A writer will show the audience through Bill's gestures and dialogue, that he's an older name. Maybe Bill has a mother, who's 96, and she plays an important part in his life.
The information i put in my posts about writing, are from my experience and I like sharing that with others. There is no hard and fast true when writing, it's based on individual, just as there is no one answer to being successful.