I grew up in a town of four thousand and change, and Charlie's pretty much hit the nail on the head. Granted, my town was in Missouri, so there are some marked differences from the Pacific Northwest (mainly in the scale of meth production and agriculture) but I can't imagine a town of this size being as prosperous as Twin Peaks seems to be.
My town originally grew because of the railroad and the coal mines. The railroad still runs through regularly, but the mines? Dried up and abandoned decades ago. There are barely any locally-owned businesses, and those that do start up close pretty quickly (I worked in a bookstore owned by a family friend for a summer, and those four months were the only four months we were able to keep the doors open).
Many people (myself included) graduate high school and never come back. Oftentimes, they go off to school and start their lives in one of the surrounding larger towns, that average about 20,000 people. (I'm a bit of an outlier with my move to NYC).
It was almost impossible to walk out your front door and not run into someone you "hadn't seen in so long!" (you saw them two weeks ago at the Rotary Club meeting and their kid tore up your lawn with his scooter because he had nothing better to do). I would be at work at the bookstore and elderly women (our main customers) whom I'd never seen before would come up to me and ask "Are you Charlotte's granddaughter? My God, you're the spitting image of her! Mildred, remember chorale in high school? Doesn't she look exactly like Charlotte?"
Needless to say, I've had some incredibly uncomfortable experiences living there.
Hearing my grandparent's stories of growing up there and seeing old pictures does give me the impression that, "back in the day", a town of this size could have resembled Twin Peaks. So possibly that's a factor?
It all returns to nothing...
You touched on something far outside the scope of Twin Peaks, but your story is not all that unusual. Small towns (not tied to larger ones) tend to see a major dip in populations in the age range of 20 to 30. Young people go off to college, military, or work. They tend to stay away through the lean years. We see a small crop of 30-40 year old people showing up as they have kids and want the safe environment or the support of their family. By 40-50 is when we really seem them return (more frequent or permanently), usually this is the start of taking care of their parents or taking over the family business/farm. Small towns in several states I've been involved with are always talking about the lost generation that happens during the 20's and early 30's.
I'll stop rambling now.