From the question, I assume that the college aged adult, 1) lives with his/her parents, and 2) lives there under the parent-child agreement (rent, and bills free). If this is the case, I do believe that it is ok for the parent to set a curfew as they still take the full care of their adult student in their home and should be able to set rules as to how they want their home to operate.
If any of these things are untrue (for example, the student may live at home under the agreement that she pays rent), then although she lives under her parents roof, the rules of how parents can expect to treat their 'child' which they are no longer fully responsible for changes, and the student should be able to come and go as she pleases.
That being said, it would be best if the college aged adult go about her comings and goings in as sympathetic and discrete a way as possible so as to abide by her parents wishes as best as she can.
Mostly, parents set curfews out of love and concern. and both the parents and the son/ daughter should acknowledge this as the starting point and preserve their relationship above all else. After all, one only gets one set of parents. And your parents love you for the rest of your life (this is true in most situations even if it doesn't seem that way).