As for your other argument - both reports count "vehicles in use" (ACEA) or "vehicles in operation" (US)
Also, the "vehicle in use" is not necessarily such a clear definition.
For instance, several of my colleagues own historical vehicles from 1960's and 1970's which are "museum registered", which here means that they must be at least 30 years old, can be used only on 30 days each year, and they are not subject to annual "MOT check" (but a different kind of check every 2-4 years).
Are they included in the "vehicle in use" statistic? I don't know. And I don't know what are the specific regulations for similar vehicles in other countries. Because the cars are 40-50 years old, even a not very big number of them would skew the statistics of average age of cars.
But anyway, these average ages may be due to little used old cars being taken out of registry, to avoid annual fixed taxes? This has not much impact on the average age of cars used in daily traffic.
FWIW, I'm from Finland, also with a history of high car purchase taxes, and the average in above data is almost 12 years. The oldest cars, however, have lower annual car tax (they had the highest tax at purchase time).
BTW I understand Denmark was about to lower the car purchase tax from 180 % to 100 % or so?