Does muscular dystrophy affect children exclusively?

NO. There are forms of muscular dystrophy that affect young adults, are milder, and less disabling. Becker's dystrophy, facioscapulohumeral dystrophy, scapuloperoneal dystrophy are examples. They are still inherited disorders, progress over time, but generally are less severe than the childhood forms of disease.
See Below. Patients with muscular dystrophy usually die in early adult hood. So, there are not adults with this illness.