I've noticed that some writers will put in the age of a character and some won't.
E.g. some will write John (40s) etc. I understand it's important to put in the age of your main characters, but do you need to do it for every character that comes into your story? Let's say your main character was a fugitive and he was being chased by two cops, you wouldn't bother to write the age of the cops would you?

I guess what I'm asking in a round about way is, which characters need their age to be specified and which ones don't?

I don't understand why some do it (40) and others write it out like JOHN, 30 it gets confusing to say the least as to why we have the differences from writer to writer.

I'm not sure but I'd assume you must do it for every character (Directors probably need it if you sell it down the line and readers like myself enjoy knowing that little bit of back story)

Maybe the most useless characters wouldn't get it like a garbage truck man who walks on-screen and disappears two seconds later.

Important character put their age, or an approximation of their age, like 30s.

Stock characters you don't have to, in my opinion. i.e.  a security guard fires a shot.  I might go with an old security guard, if I want to put him in an age bracket.

As for parenthetical age (30) versus writing out, John, 30, is a matter of preference.  I've seen it done both ways and you'll likely get arguements on which is better.  If it's even worth the trouble.

I'm at the point where I'm doing more age ranges, like 30s, 50s, etc.  An exact age might be important in some stories, but otherwise I figure a range gives those in control more flexibility in casting.

I usually put that a character is about a certain age, although I occasioanlly specify an age. I figure it's all subjective anyway, since you can't tell how old some people are anyway, so if they look somewhere close, that's just as well. They're going to cast that way anyway.