In terms of promotion, the gambling company may change its strategy in using these celebrities or use an age limit for those allowed to see these advertisements.
But from another perspective, it doesn't seem easy to do because children under 18 years old may already know how to see the ad. But indeed, the problem of gambling addiction is a very serious problem that needs everyone's attention. They can play gambling but with strict terms and conditions.
They have zero arguments against this law at the moment! Celebrities like already mentioned Christiano Ronaldo, have a huge impact on the young generation. Look, kids try to wear their boots, t-shirt with their names and because of them they support the clubs where they play. A lot of kids are dreaming to become as successful footballer (Soccer) as Christiano Ronaldo.
Also, estricting the ads to those aged 25 and over for most sites is a good idea too. I personally think that 18 years old aren't adults, most of them can't live without parents support, most of them change their mind very often, people in this age don't even know which profession to choose and some of them re-enter the university after finishing the one or are unhappy with their career.