On the other hand, the value of the reviews on these sites goes down the older they get.
On Amazon it seems like the opposite is true. Because of their meta-review process, where users can flag reviews as helpful or not, it seems like old and classic reviews tend to float to the top, while recent ones have a significant hurdle to overcome before they make it to the front page. See for example here: http://www.amazon.com/gp/product/0262011530
(Admittedly, users can also be fickle and support other kinds of reviews for different reasons, cf. http://www.amazon.com/Tuscan-Whole-Milk-Gallon-128/dp/B00032...)