A six-week pilot began in 11 stores in Armadale, West Lothian, at the end of March. The ban will be in place on Friday and Saturday nights, from 5pm till 10pm.
The project is a partnership between Lothian and Borders Police, West Lothian Council, and retailers, including Spar chain Botterills and Scotmid Co-op. It is the first of its kind in Scotland and follows a successful initiative in Cleveland.
Anyone who looks under 25 will be asked to prove their age.
All off licences in the town have agreed to participate, but some have expressed concern over the scheme's value. Botterills operations manager Brian Straiton told Convenience Store the chain backed the campaign, but called for a more rounded approach to tackling underage drinking, including fining the parents of teenagers found drinking on the street.
He added that the weekend ban could lead to confusion among staff, who already operate a Challenge 25 policy. He said: "I don't think this is the ideal approach, but we will back the initiative 100%."