Could an adult have pandas?

No. Pandas is an acronym which stands for pediatric autoimmune neuropsychiatric disease associated with streptococcal disease. No. By definition, pandas is a pediatric disorder. It is possible that adolescents and adults may have immune mediated ocd, but this is not known. The research studies at the nimh are restricted to children.
Maybe. Though pandas is by definition pediatric, if the onset of the ocd occurs in childhood and persists to present day with a waxing and waning course similar to pandas, then pandas may be present. I have seen this in adolescent patients (18 - 19 years), though have not seen this in older adults because i only see pediatric patients. If the history fits, then i would seek a pandas evaluation.