My advice would be to seek the opinion of a board certified veterinary dermatologist.
Collies have several different skin problems that can pop up at any time.
Around the age of 5-6 mo., when the pups are going through puberty more or less, some lines tend to break with demodectic mange, under stress and during this age. As far as demodectic mange here is a bit of info. A bit of info. Lesions consist of erythema, scaling, alopecia, ulceration, crusting, hyperpigmentation, and hypopigmentation. Lesions develop in the skin of the face, ears, lips, periocular areas, and tail tip and over bony prominences of the distal extremeties.