In April 2022, a clinical report was published in Pediatrics, a publication of the American Academy of Pediatrics. These pediatric guidelines are "designed to assist the pediatrician in caring for the child, adolescent, and family in whom a diagnosis of Down syndrome has been confirmed by chromosome analysis or suspected by prenatal screening."
Full text (no cost)
There is also an AAP Guide for Families.
For adults, please see the GLOBAL Medical Care Guidelines for Adults with Down Syndrome.