I think online casinos does not bother about gamblers age at all. That is the internet, everyone are used to giving and receiving false information. There is no use even trying to prevent gambling because how can authorities also check if underaged or an adult has gambled. And if something goes wrong during session or an issue appears, what could underaged gambler do? Try to sue the casino? The case will be lost in 1 minute in court, plus underaged parents will probably get a fine or a warning for bad parenting.
Casinos allow underaged to gamble, because they are not as experienced as adults in gambling, they will lose faster, and even if they win, with their small deposits, withdrawals not going to bankrupt casino. But if they win jackpot, KYC will save casino from paying.