Ah, ok. Maybe steam will implement such an age check now that a fairly significant market is unavailable to the publishers, One can at least hope.
They won't. Legally selling digital USK18/M-rated stuff to underage kids is a huge part of their business model, because physically they can't buy it in Germany. Online? No problem! These teenagers don't care for old banned stuff made before they were born.
But if digital stores ever implement a real age check, it has to apply to all games! So were will never be an age check ever, because it costs sales.