Officers from the Royal Borough of Windsor and Maidenhead, supported by the Gambling Commission and Trading Standards, conducted age verification test purchasing at Royal Ascot.
Out of the 17 operators tested, seven allowed a 16-year-old to place a £5 ($6.10) bet and now face regulatory action from the Commission.
The Commission has not yet named the operators under review.
Richard Watson, Commission Executive Director, said the on-course sector had a history of failing age verification exercises.
He added: "Pass rates have failed to meet the standards expected and the sector has consistently performed to levels below those we see in other gambling and age-restricted products.
"By way of example, over the past four years, the on-course sector has a pass rate of around 35% for Think 21 testing."
Councillor David Cannon, Lead Member for Public Protection at the Royal Borough of Windsor and Maidenhead, said: "It is not acceptable to take bets from young people and I am pleased strong action is taking place.
"Our hard-working team of officers will often carry out test purchasing across the borough to ensure our residents are kept safe and traders are abiding by the rules."
