the only sinuses that are really developed by that age would be the ethmoid sinuses located on each side of the nose and these rarely become infected. the major paranasal sinsues (the frontals and maxillaries) do not fully develop until the early teen years. i can't say i have seen an 8 month old with true sinus infection in 40 years of practice but i have seen other clinicians diagnose one clinically. it would be highly unlikely for an infant to have a sinus infection (confirmed by an imaging study) but theoretically i guess it can happen.
can an infant under 8 months of age gat a sinus infection?