So I don't remember anyone hiding it or an age where I suddenly "discovered" that there had been a Holocaust. Yes, the more graphic details I probably found out around age 6 or so, but Hitler's name was cursed regularly in our household, we didn't buy German products and the Battle of Britain was vividly recalled as the knife edge on which our family's existence had teetered, had it not been for the Royal Air Force gumming up Hitler's invasion plans our family would have met the same fate as our continental European cousins.
I think in the modern West there is a desire to shelter our children from all evil, to cocoon them in safety and love, hide the horrific ugliness of the world from there. I'm not advocating showing graphic details of atrocities to little kids, but sadly there is evil in the world and our children need to know about it, will catch glimpses even when their parents try to keep it secret from them.
Better we the parents be the ones to teach them about it, and yes, even from a fairly young age. Simplistic basic facts at first, not much detail, but kindergartners get the idea of superheroes battling evil, they can grasp the idea of a wicked man trying to hurt others and good people fighting him and eventually stopping his evil plan to destory the Jewish people. Yes it is a scary thing to learn, but they don't have to face it alone.