Yes Down Syndrome is a developmental disorder.
Developmental Disorders or Developmental Disabilities are problems that are life long, severe problems which can be either mental disabilities or physical disabilities.
And since Down Syndrome is permanent and never goes away and affects a persons life and the ability to function properly in the world then that means that Down Syndrome is a developmental disorder and a developmental disability.
Developmental Disabilities and Developmental Disorders can also be being blind, learning disabilities and much more.
if a person has mental disabilities and the disability is affecting the person long term and for life then it's a developmental disability.
Down Syndrome is with the person for life and never goes away.
You can tell someone has Down Syndrome because the persons face looks a lot different than other peoples.