Unlike proposed new laws in Scotland, the ban will be voluntary, but councillors hope it will be enough to reduce public disorder in the borough.
In the press, Croydon councillor Steve O'Connell said : "We want to tackle the corner shops that are selling eight cans of Stella for a fiver because it can lead to young people getting involved in anti social behaviour."
proposal was backed by London Mayor Boris Johnson, who told the capital's assembly the idea was "a very interesting one".
He added: "Where we have got particular problems in particular areas, off-licences and supermarkets should stop the sale of alcohol to the under-21s.
" Boroughs can take a lead and banish the scourge of binge-drinking."
WSTA chief executive Jeremy Beadles said: " It's an utterly illogical idea that 18, 19 and 20
olds should be able to buy from a pub, bar or restaurant or drink at home entirely legally, but shouldn't be able to buy alcohol from an off-licence.
"All you're doing is encouraging people to shop outside the law. I don't think it will make the blindest bit of difference and it's demeaning to the vast majority of 18, 19 and 20