Human Keratinocyte are easy to cultivate when the donors are young. Older donors = more problems in cultivating of keratinocytes. So far my experience. But Gerson is right, whats your question?
we planed a project to reprogrammed keratinocytes to iPS. But we have many older patients and problems with cultivation of these keratinocytes. So we don't do this project. I think best paper about this is this one: http://www.nature.com/nbt/journal/v26/n11/abs/nbt.1503.html