If you look at a cross section of an EPDM roof it has a very thin top layer of white, the substrate is black. The white top coat will slough off do to sun and rain showing the black substrate. 15 years is not old for an EPDM roof, I would not replace it unless there was severe damage. Coating it will return the white color and make it cooler inside and protect it for another 10 years before it wears off. My roof is 23 years old, I coated it with just one coat of acrylic coating in 2007 and it is wearing off. When it all wears off I may coat it again. The roof has never leaked.
