To determine at which ages providers choose to screen for mental, behavioral, and developmental disorder/delay (MBDD), and what they find; and which, if any, public and professional guidelines are most effective at identification.

Naturalistic retrospective cohort study of 215 general pediatric and family practice clinics within 24 US states involving 160 634 encounters during which MBDD screening tests were administered.

Almost all clinicians (96%) administered screens at ages targeted by the American Academy of Pediatrics (AAP), that is, 9, 18, 24, and/or 30 months of age, but also at younger and older ages: 57% opted to screen at ≥5 years of age. Of the 8% of children at risk for probable MBDD, 27% were detected at American Academy of Pediatrics-targeted ages—71% across the birth to 5-year age range and an additional 29% at ≥5 years of age. Children >30 months of age were 3 times more likely to have probable MBDD than were younger children, and those >5 years of age were almost 4 times more likely to have probable MBDD. Older children were more likely to have psychosocial risk factors, but age itself was the most powerful predictor.

Most clinicians preferred to screen across the birth to 8-year age range and their findings revealed that most MBDDs cannot be detected in the earliest years of life. Policies regarding the timing of screening should be expanded to include all well visits and between visits if needed.