They teach far too much unpleasantness too young in my opinion. Great Fire of London to six year olds, including building and burning models! No, thank you. My own child only heard about that one from a friend who'd done it and it still caused them months of nightmares about people being burned alive.
At 13 the detailed horrors of WWII were still too much for them, as was going round an old prison that was graphic about conditions. They love history and know so much about it, but also have a surfeit of empathy and a very vivid imagination. We home educate so have more control, thankfully, because I think they'd have needed to come out of a lot of lessons and school would have ruined their love of history.
Horrible histories has been great for us, but starting around age 8, and skipping over bits that were too much, plus watching it together for a few years so we could discuss things and deal with things as they came up. Other than that only time and the maturity to better cope with their high empathy have helped.
My younger child hasn't had the same level of reaction, and copes well with things like operation ouch, which the older one also found too much. It's very much innate to the child and imo needs to be respected.