A very interesting topic, those discussions from both sides are great. Keep it rolling!
As for why (Western) European fleets are generally newer than American ones (although that is now rapidly changing), here is what I think:
1. Smaller fleets to replace - the average European airline has 60-100 planes in total, some are actually smaller than that. US airlines have 250-600 or even more planes. Think about it, if a company like Coca-Cola tried to replace even 300 trucks in a short time that would be a costly move. Replacement of even 50 planes at a time is even more costly.
2. International routes - the European airlines have smaller home bases so that international traffic is more important. In large part, therefore, these airlines competed heavily with one another in promoting rival countries and hubs as European gateways. To attract that traffic the airlines placed a premium on service and an overall good image, into which new aircraft fit perfectly.
3. Subsidies - like it or not, the fact that these airlines have been heavily State sponsored (or owned) has facilitated their plane replacement and their losses in other departments. US airlines do not have that comfort back-up so they kept the older jets, which were paid for and thus earned profits, for longer.
4. Airbus - again, like it or not, Airbus has many European participants and so encouragement of the airlines to buy Airbuses actually boosted local economies by enhancing production. BA and SAS have been notable exceptions to this rule, being strongly Boeing and MD respectively.
Eastern European countries have had greater financial difficulties and so all-new fleets are not the rule there. However, many of their airlines have bought or leased western planes such as 737s, 767s, 757s and some Airbuses (LOT and Malev are purely Western now) in order to serve Western European ports which became reluctant to continue handling the older, noisier and more polluting Soviet types. Additionally, it again became an image thing as the airlines strove to capture the Western tourist market for their respective nations and thus the need for western planes, with which western flyers felt more comfortable as a rule. Some of the carriers still use Russian types on the lower-profile routes within Eastern Europe and the CIS.
High profitability and the desire to present a high-profile image are the reasons that some Far East airlines like SIA and some Middle Eastern airlines like Emirates and Gulf Air have brand new fleets.
US carriers, with their many domestic destinations scattered into complex webs of services, generally require large numbers of planes at any one time to ensure that the entire networks can be operated at all times. Thus, while their fleets may be large very few planes are actually surplus to requirements. This, plus an increasing demand for flights, has meant that 727s have lived on despite the introductions of replacements like 757 then 737-400 and only now, with 737-800 and A320 coming in, are they actually being retired.
A closing point here - while a European airline may fly from its base to, say, Heathrow 10 times daily, Paris 10 times and other regional ports 3-5 times daily on average and fly different long-haul routes on different days (with the exception of the majors like JFK, ORD, YYZ and possibly NRT), US carriers fly to almost every destination, local and international, several times daily and may have 15-20 flights daily between many city pairs. That difference in scope can easily account for the need to keep all those planes flying.
So, it is not a purely environmental matter here, although both sides have been introducing tougher regulations on noise and emissions and airlines on both sides have been adapting to them.
Hop to it, fly for life!