Merseyside chief constable Bernard Hogan-Howe said stopping stores selling drink to under-21s would help cut binge drinking and alcohol related violence.
"My suggestions are to restrict the sale of alcohol to young people through off-licences and to come down hard on adults who go into off-licences to buy alcohol and hand it over to youngsters in the street," Hogan-Howe said.
The WSTA has criticised the proposal. Chief executive Jeremy Beadles said: "There is no evidence to show that allowing 18, 19 and 20-year-olds to buy alcohol is in itself any way irresponsible."
Raising the legal purchase age would not protect shop workers from being intimidated by young people, he added.