Retailer Spar will adopt a "Challenge 25" policy from March 1 to tackle underage drinking.
The company's current policy requires staff to ask for ID if they think a customer trying to buy alcohol is under 21 but this will be increased to 25.
It claims it will retrain its 2000 staff before the new policy is adopted.
Chris Endersen, a Spar controller said: "It is paramount that staff are trained to remain vigilant and to have robust procedures to detect and prevent potential underage sales. It is also critical for us to support them as they bear the brunt of this policing."
By September 2009 all senior manager should have the SQA qualification from BIIAB, which is the qualification required before a personal licence can be held under the Licensing (Scotland) Act 2005.