So how does the US deal with age restricted games? I find this much more related to actually willing to implement a rule, rather than having rules for the sake of it (like the US buying alcohol rule - it is forbidden for people under 21 to drink but 40% of the people between 18 and 21 drink ?! source: https://en.wikipedia.org/wiki/Alcohol_consumption_by_youth_i...).
Also what does this have to do with anything? e.g. adult-only games are simply unavailable on Steam in Germany. It doesn't matter at all how old you are.
It means there is a law, and Germany makes a reasonable effort to apply it. Germany asks for Steam to use a different system than "Asking the age to the user" (which in my opinion is hilarious and obviously a joke), Steam decides to not sell the games. (source: https://www.heise.de/en/news/Steam-Payment-providers-force-V...)
It's not like "Germany wants to ban sex games altogether", which seems to be what some other private groups would prefer.