Their conclusion: Grayer is greener,

[Plane]

In an effort to improve models of global warming, a team of researchers led by Brian O'Neill of the National Center for Atmospheric Research estimated (PDF) what U.S. emissions would look like in two different cases: one in which the age structure of the population looked exactly the same as today's, and another where—as many demographers project—the percentage of Americans over 65 more than doubled.

http://www.slate.com/id/2199575/?GT1=38001

They also conclude that older is poorer.