I wasn't all that surprised when I heard about the Gore separation after 40 years of marriage. After all, he has launched a new career, one outside of politics that takes him all over the world. She no longer needs to be at his side and has likely been pursuing her own interests for the past 10 years. Growing apart would be natural. That they chose not to remain married in light of the new reality of their lives is probably unusual, but so are their lives.
Most people in their 60's are slowing down, cycling out of work and into retirement, not ratcheting up into new careers. I know statistically that divorces later in life are initiated by men when they find a new partner (said not to be the reason here) and by women when they are "tired of putting up with stuff." I suspect that having the financial means to live separate lives and the ability to see another 20 years of life ahead of them makes it look like a good enough reason to go their own way.
Much has been said about the fact that "until death" marriages were one thing when life expectancy was shorter and asking a lot of two people with the current longevity. Perhaps we need to look at later in life divorces not as a failure of a marriage, but as a good run and a new opportunity to live the later years either on our own or with other partners. Forty years seems like a good long run to me.