With a euphoric clinking of glasses and a daring sip of Mai Tai cocktail. 18-year-olds entered the ranks of legal drinkers in Massachusetts this week.

The immediate impact was felt both at local bars and at retail stores, but the long-term effect will probably center on retail sales because of the costliness of bar-hopping.

When the age limit officially dropped from 21 to 18 at midnight Wednesday, the paragons of loud music and hip conversation--Jack's, Charlie's Place, and so forth--were swamped by ecstatic teens and bearded college sophomores trying not to look the part.

Business at several bars around the Square boomed between midnight Wednesday and 1 a.m. Thursday. It was double the usual at Charlie's Place, where a customer had to help wash glasses to keep pace with the newly-enfranchised clientele. Long lines have backed up traffic at Jack's every night since the 18-year-olds moved in.

One problem now may be identifying under-age drinkers. David Gross, the bartender at the Casa B, summed up Thursday, "I've gotten to where I can tell pretty well if someone is 21 or not, but I don't really know how an 18-year-old differs from a 17-year-old."

Sales at retail stores have also risen in response to the change. Martignetti's in Brighton recorded a 5 per cent sales increase Thursday, and the Harvard Pro reported an increase sale of 100 bottles yesterday--mostly beer and Gallo wine.

One retailer, asked if prices had gone up in response to the 18-year-old drinkers, yawned and said, "Oh sure, the Mafia ordered them all up this month." Regardless of that claim, the liquor business is swimming.