Who cares how old they are?? Does that really have an effect on their music?? As far as I can tell, there are only a few reasons for changing the way you sound as a band.
1. You are tired of playing the style you have played
2. Your record company told you to
3. You want a bigger piece of the demographic
There are probably more...but out of these 3 I think that Metallica wanted a bigger piece of the demographic. We all know by now that Metallica is largely driven by money and how many times over they can become millionares!! They still had a huge fanbase before Load!!! Why did they change their style?? To appeal to the younger crowd!! All kids want to hear nowadays are the sell out Staind like bands. It all started with Limp Bisquit...look at them now!! If Metallica would have kept their original musical style (who cares about how their hair is cut ect...that is just marketing tools to make money) they would still be hugely popular today! Did Led Zepplin ever change styles?? All of the classics that are still popular today kept the same style, explored different creative ideas, while still maintaining their millionare status. Metallica is just a bunch of greedy sell-outs who will eventually be forgotten just like Guns-n-Roses. Barely anybody listened to their new stuff because it was crap (after the 2 disc album). But Appetite for Destruction was amazing.