If online casinos were really checking and trying to ensure that a gambler behind the screen is an adult, that business would not be so popular and profitable today. As long as people keep bringing and losing money, the casino does not care about the age of a gambler. They only start to care a little when gambler tries to withdraw large amount, his gambling session is short, he isnt much active on the platform and his playstyle is suspicious. Why online casinos does not bother about age check also, is because there is no strict supervisory authority and casinos licenses does not cost a lot.