All game companies operating in China, software and hardware, have to go through the CCP. They could definitely force companies to add software timers that automatically shut games down when a time limit is reached.
Underage users would find ways to appear to be 18 or older. Or use multiple accounts to continue playing. This is already common in many grind games. You have support accounts that feed the main account items.
Or just straight up hacking the timer system. Or playing games published outside if China.
Where there's a will, there's a way.
I'm sure some people will find ways around it, but I don't think that will be the norm.