Actually, DM is fairly common in young adults although older folks get it as well. I have polymyositis but lead a local support group for myositis diseases in the Chicago area and am quite familiar with the disease. I suggest that you post on the following sites and you will find many with DM who can answer your questions.
www.myositis.org
A closed group for DM and PM on Facebook. Just request to join and you will be admitted...it is open to anyone with PM or DM.
https://www.facebook.com/groups/201461863232077/
Let me know if you have any specific questions.
Bill