Alexa rankings are not nearly as precise as Google Analytics. Although I'm not quite ready to make public the analytics results I can say that broadly speaking the differences you see on the alexa rankings underestimate romanreference by a measure of half (romanreference has actually 450% more visitors than parisreference (not just 200%) and 1,300% more than copehagenreference (not just 540%)).
Broad differences do reflect analytics measures from there on though, with paris going some 250% faster than cph and copenhagen going ca 150% faster than citiesreference. Major difference otherwise is that Berlin and Barcelona are actually 100% bigger than the other younger websites.
Looking at the analytics each website visitor's size, broadly reflects its age online with the only exception that to get really close to the precise measure you should double the value. For example RomanReference is 250% older than ParisReference, but ca 500% more valuable in terms of visitors and 1,000% in terms of revenue, ParisReference is 150% older than CopenhagenReference but 290% more valuable etc.
Age does matter for websites' visitors and market value. For a reason.
Let's look at another public measure, listings
Roman Reference as of January 2010 has 455 apartments
Paris Reference 77 apartments
London Reference 28 apartments
Barcelona Reference 19 apartments
Berlin Reference 10 apartments
Copenhagen Reference 9 apartments
Madrid Reference 9 apartments
Miami Reference 8 apartments
New York Reference 8 apartments