Think about it ... If no new building materials and energy resources have to be used in a home, we are one step ahead. 
More and more, we see renovation companies thinking green and helping homeowners improve the energy efficiency of their older homes.
This can mean more affordable homes for homebuyers who recognize a good thing when they see one and know they can upgrade the home they buy at their own pace.
Green does not always have to mean new. And think about where all that material goes when we tear down older homes, and the accompanying costs to transport debris from a building site.
Older homes have some benefits as well ... mature trees.
I had lost a 100-foot pine (split by lightning), only to discover that it was placed perfectly as a wind block. So aside from losing the beauty of the tree, I now have the wind beating against my home.
Sometimes, you find surprises too ... like learning those lovely old windows do not have to be replaced because you have the original storms. The air barrier created between the window and the storm is energy efficient ... just make sure that caulking is still in tact.
Once you buy an existing home, look at those parts of your home that can be made more energy efficient, for example,
- insulation,
- windows and doors,
- mechanicals,
- appliances.
- caulking,
- and landscaping.
And an added perk ... you may have just saved a bit of architectural history :)

Cornwall, New York
Marie P. Meyer, Light Realty LLC, Serving Orange County, NY
I like that title... Me thinks a lot of environmentally friendly folks want to be environmentally friendly from their NEW home!