Pubs and clubs in Sheffield passed a test purchase operation where children were sent in to try to buy alcohol.
Police sent youngsters into 15 licensed premises in the city centre and said all staff working in the pubs and clubs passed the test.
Chief Inspector Jayne Forrest, who is responsible for policing in Sheffield City Centre, said: “It is really pleasing to see that all 15 of the licensed premises we visited passed.
“A number of similar operations have been carried out before and we’ve worked with local licence holders and staff to make sure they are aware of the rules and feel more confident in checking identification.
“Underage drinking affects the wider community in that it can often lead to anti-social behaviour and there is also a risk to the health and personal safety of that young person.
“South Yorkshire Police will continue to work alongside partner agencies to ensure staff at licensed premises keep up the excellent work and to continue challenge anyone that does not look 25-years-old before they make a sale.”
Letters will be sent out to the bosses of pubs and clubs where their staff passed the test.
A South Yorkshire Police spokeswoman said: “Test purchase operations are regularly carried out throughout South Yorkshire and similar checks are being planned for the future.”