Valve games literally target mostly the 13-18 age range...
Also do you have any fucking idea what the M rating means...? M RATING IS STILL MINORS.
It's 17 and up, 17 IS A MINOR.
The ONLY rating that's for adults is AO or adult only 18+. Real world gambling is literally only legally allowed in AO games.
If valve wants to have gambling then their games should be AO. Which then carries a legal requirement of age verification already and has for decades.
If you have gambling in a m rated game that involves real money you have underaged gambling. It's literally cut and dry