From Oct 1, it will be illegal to sell cigarettes or tobacco to anyone under 18 and shopkeepers who fail to put up signs informing customers of the change face fines of up to ú1,000.
Signs must be at least A3 in size and state: "It is illegal to sell tobacco products to anyone under the age of 18" with letters of at least 36mm high. The signs must also be in a position visible to customers and staff.
The Association of Convenience Stores is also advising retailers to tell customers about the coming change to help avoid confrontation when refusing sales to 16 and 17-year-olds.
A statement said: "Since the age change could prove unpopular with young smokers who will have their right to smoke taken away, it is important that retailers start to inform their customer of the law change in advance."