. How many years (range) does it take for a person to develop Severe Obstructive Sleep Apnea (OSA)?

It can develop at any time -it is a function of upper airway obstruction-not time

2. How many years (range) can a person have OSA before it is recognized and diagnosed?

The suspicion of OSA - has to do with excessive daytime somnelence being identified, a spouse or bed partner expressing concern over snoring and gasoing for air by the individual and it is only "confirmed" by a sleep study.

3. Between 1991 thru 1999 were all Physicians, Physician Assistants and Nurse Practitioner in both the military and private sector fully aware and educated on how to recognize and diagnose OSA based on patients symptoms?

The field has undergone a significant amount of change - many of the objective parameters such as the
Epworth Sleepiness Scale

Standards for performing polysomnagrams

were only recently developed.  The medical complications (heart, endocrine) are still being appreciated so the seriousness of the situation has really only been realized in the last years and only in the last 3 years has there been an established board exam for professionals.

4. Why is OSA still underdiagnosed in this day and age? 

One reasonable assumption is that the average primary care practitioner does not question patients about sleep in routine evaluations

5. Between 1991 thru 1999 was OSA symptoms often overlooked and misdiagnosed?

Already answered

6. What age range do people develop Severe Obstructive Sleep Apnea?

Can happen in kids, teens young adults thru seniors