Yes, you can give them to the adults as well.
Sometimes the feed has not been stored in a cool and dark place or the milling date is way past., meaning the feed is old and no longer providing the necessary vitamins etc.
This could be due to a vitamin deficiency (malnourished parent birds).
Usually Thiamin and Vitamin B12, so providing these vitamins should help.
Here is more info:
https://www.backyardchickens.com/threadloom/search?query=vitamin%20deficiency%20ducks&tab=620