The young or old aspect I'd say depends on how much that country cares about and plays up its past. Take Turkey or Cyprus for instance -- these are countries constantly referenced in biblical texts (see: Chittim), full of ruins, treasure troves of empires have passed through and left their mark there, but the people - for the most part - simply do not care. There aren't that many museums dedicated to ruins there, except maybe for the tourists who seem to care more.
Germany takes huge pride in its medieval castles and these are plain to see by their upkeep and their prominence on wine bottles. There are a few references to what happened between 1936-1945, and though there is (was?) a culture of atonement/regret/guilt in the immediate generations that followed after, it's one where you really wouldn't know what happened unless you tripped over a gold stone with someone's name on it.
In the UK they play up the Kings&Queens during history classes, but completely skip over the role that the UK had in the slave trade (and also it's role in abolishing it).
My point: It's a selective memory these European/Eurasian countries have. Yes the US has a short history, but every single aspect of it is seemingly brought into the forefront and talked about in great detail, warts and all
Intesting, I agree with you nation memory is always selective. Russian in particular have a very paradoxical relation to the sovietic era : everyone knows about the Gulag but also their is a strong nostalgy for this era.