All but one of the 27 stores tested in Workington, Maryport and Cockermouth refused to serve a teenage volunteer with alcohol and fireworks.
Police officer Alison St Quinten said: "The message seems to be getting across. People are asking for identification. It is in the forefront of people's minds to ask customers' ages. It was a good result.
"We did the exercise as we were receiving complaints about youngsters getting alcohol."
John Greenbank from Trading Standards added: "Two or three years ago there was a 20% fail rate in test purchases. Now it is more like 5%, so there has been a big improvement."
The news reflects ACS chief executive James Lowman's claim that c-stores are leading a change in attitude towards ID checks for age-related purchases.
Questioned by the Culture, Media and Sport Select Committee, Lowman said: "We are bringing about a culture change in the trade here, where you are increasingly seeing people being challenged for proof of age."
The Retail Alcohol Standards Group, of which ACS is a member, has announced its members will adopt a Challenge 25 policy from 2009.