I believe the first number after the letter is the model year. So if it's D7XXXXXX it's a 2007. I've always bought my Dahons on price rather than model year, usually a discounted old stock 1 or 2 year old model. When I've sold them a couple of years later - as long as they've been looked after they've always fetched almost what I paid for them. Keep the docs and the touch-up paint.

The serial number is under the bottom bracket. You may have to turn the bike over to find it. My current bike is a 2008, bought in December 2009. I saved about 25% on price.