Here's why I think Dave is pushing against vagueness. First off, he's talking about openings
, the first page or two of your story, so this is about transport. Transport is a big thing for Dave. Your story has to sweep him out of the real world and into the physical, emotional and intellectual world of your story.
Physical transport is about setting and description. It's about the five senses, which Dave has also covered in his Kicks. It's about giving the reader a strong, clear, fully developed sense of where the story takes place.
Is your character surrounded by trees? Then make sure the reader can see, smell, hear and feel the forest. Is the forest young or old? Towering lodgepole pines at 5500 feet, or dank cypress trees festooned with moss? Those two forests not only look different, they smell different and sound different. The ground underfoot feels different. The air tastes different.
Don't just say "surrounded by trees." Put the reader fully in the forest.
Emotional transport is usually about characters, and vague language tells the reader that the character doesn't really know or doesn't really care, and both of those are distancing. For the girl who was about 12, if my character was expecting someone who was 21, I'd think he'd care that the girl was actually 12. Would he be shocked, surprised, horrified, intrigued? I want to hear it in the language.
The girl was 12 at most, that no-man's land between childhood and adolescence. Too old for Barbies and pigtails, but too young for the lacy camisole and high-cut denim shorts they'd dressed her in, too young for the red that painted her mouth or the dark kohl around her eyes. And far too young for the jaded look the kohl could not conceal.
This gives me so much more than an approximate age. It gives me a mood and a description, it tells me how she feels and how the MC feels about her. It even gives me something of world, a place where girls wear pigtails and play with Barbies even though this one doesn't.