This Cat scan of an adult shows the air of the sinuses in black, the bones in white, and soft tissue in gray. Notice that there is a very thin air passage. This is an example of an anatomic variation that can predispose to sinusitis. Even very slight swelling of the lining of the passage will stop normal ventilation of the maxillary sinuses. Adults are more likely to require surgical treatment than children, but even adults can usually be helped with medical treatments.
Adenoids are almost never a factor in adults.
Most kids get a sinus infection at some point. Usually, these clear without even the need for medical treatment. Of those who do need treatment, most kids get better with a simple course of antibiotics. Those kids who do not improve with antibiotics usually will improve with a few weeks or months of combination medical therapy.