If people work to stay slim, can they be reasonably certain to avoid getting sleep apnea?

Yes, with provisions: Certainly the biggest risk factor for developing obstructive sleep apnea (osa) is excessive weight (bmi >25). Having great health habits like adequate exercise (30 min 5x/week), appropriate caloric intake, adequate sleep (7-8hrs/night), and significant social interactions will also combat most of the other risk factors. The other risk factors are mainly due to birth defects, or other lung diseases.

No: Body weight is but one risk factor for sleep apnea. A significant number of people with sleep apnea have a normal bmi. The risk of sleep apnea increases with age regardless of weight. In brief, your chances are better if you stay slim, but you are far from certain.

Usually, not always: Most apnea patients are over weight. However, I have many patients who have severe apnea and they are thin. If you have an obstructed airway, nose, tonsils, palate, uvula or tongue, you can have apnea regardless of your weight.