When MMD1 begins in early childhood (but not at birth), the first symptoms tend to be more behavioral and cognitive than physical. Some people with the juvenile-onset form struggle with planning ahead, decision-making and visual-spatial processing. An "avoidant" or apathetic personality can be part of the picture.
As the child matures into adulthood, typical features of adult-onset MMD1 usually emerge.
To learn more about the juveile-onset form of MMD1, be sure to read Juvenile-Onset MMD1 Can Cause Cognitive, Behavior Challenges  (part of Quest's In Focus: Myotonic Muscualr Dystrophy  series).