DE isn't effective when it comes to mites (believe me, after a 6 week battle with red mite I know!) and red mites don't stay on the birds but hide out in your coop during the day, feasting at night. So you need to identify what you are dealing with to determine the best course of action as it could also be lice. Wipe under the roost bars to see if you come up with blood streaks/squished bugs - that's red mite. Do your birds free range or come into contact with wild birds or rodents? Those can carry parasites that end up infecting your birds. If you are in the US you should be able to get permethrin spray to deal with external parasites.
Here's a good website for identifying external parasites:
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
Young birds shouldn't molt until they are around 18 months old, though I had one pullet who did at just on a year old.