I think what is going on is a combination of relatively good testing capacity from the beginning and mostly young people infected, which due to the testing were prevented from spreading it to older demographics. See https://experience.arcgis.com/experience/478220a4c454480e823...

Also there is the purely anecdotal hypothesis of a cultural difference between for example Italy, Spain and Germany. In the southern countries it seems like older people have quite an active social life compared to Germany.