It's specific to each couple and the reasons for why they think it's time to end the marriage. I've had a semi-gray divorce after two decades of being married. It's also the realization that life is too short to be spent being unhappy. If the two people do not work towards making things better then it may be better to part ways. I like the idea of "living apart together" but as we get older I do think it may not be a sustainable option for financial and health-related reasons.