I recently had an x-ray read as mild prominence of the interstitium. an x-ray was found from 7 years ago with markings, but slightly less prominent. if it was fibrosis, could it take that long to develop? lung function tests and o2 sat are normal?

You've had pulmonary function testing which was normal. Have you been evaluated and advised by a pulmonologist? I would ask if a chest CT scan, arterial blood gas determination, exercise tolerance test would shed any more light on the degree of your interstitial lung disease. The cause is elusive. If you haven't consulted a pulmonologist as yet, do so as I think you'll need periodic followup.