If you see or suspect any licensed premises are supplying alcohol to those under 18, report this to the local Police or licensing inspector at your local Council.
You could also record any suspected underage purchasing in a community incident log.
Work with your local health promotion team (District Health Board/Regional Public Health Service) and other interested parties to mount a campaign aimed at ensuring licensed premises in your area comply with their obligations not to sell alcohol to minors.
You could also raise awareness of the need to reduce the supply of alcohol to young people in your community.