Merlot should be served at room temperature. The more-structured, less-fruity styles, especially Bordeaux, will benefit from being poured into a decanter and allowed to breathe for up to one hour before serving.
The majority of Merlot from New World regions (Australia, United States, South America) is meant to be consumed within a few years of vintage date, although the more expensive wines can last for many years.
Buying (and drinking) an older bottle will typically get you a mellower softer wine with less obvious fruitiness and more herbaceous aromas. A younger wine will offer brighter, juicier fruit flavors and a firmer texture in the mouth-feel.
Store bottles in a cool, dark place. While it is now common for wine makers to use plastic corks or screw-tops lids which may be stored vertically or horizontally, those with the traditional corks should be laid on their sides to keep them moist.