Dermatomyositis (DM) are rare and heterogeneous systemic autoimmune diseases, characterized by the association of muscle inflammation, skin inflammation and vasculopathy. DM concern both adults and children. DM can be life-threatening (interstitial lung disease, infectious complications) and responsible of significant functional disability (muscle weakness). Age of onset appear to be an independent prognostic factor. Juvenile-onset DM is characterized by a higher frequency of calcinosis, skin ulceration and digestive vasculitis. In adults, interstitial lung disease and cancer are more frequent with higher mortality. Data concerning the comparison of the initial severity between juvenile and adult-onset DM are limited. The main objective is to compare global severity between juvenile DM and adult-onset DM at initial diagnosis. Secondary objectives are: * to compare organ-specific severity between juvenile DM and adult-onset DM at diagnosis. * to compare damage during follow-up and at last follow-up between juvenile DM and adult-onset DM. * to compare activity at the last follow-up between juvenile DM and adult-onset DM. * to compare iatrogenic complications between juvenile DM and adult-onset DM.