Steinerts myotonic dystrophy (DM1) features significant phenotypic heterogeneity and adult-onset presentations, meaning the condition can first develop or be diagnosed after the age of thirty.
The retrieved literature consistently notes that myotonic dystrophy type 1 includes an adult-onset phenotype. Specifically, clinical reports document adult diagnoses and presentations occurring well past the age of thirty, such as in parents diagnosed after their children or individual case reports of patients in their forties.