Na man, this is the thing that really gets me on Metallica. They were the KINGS - the most popular of the "big 4" thrash bands at the time. Then they made the black album, which as I've said isn't a bad album and I don't mind it, however they enjoyed huge commercial success outside of metal. So they keep making albums that enjoy commercial success outside of metal. Then they get to a point where they want to go heavy again. Get "back to the metal". I think ok, I don't blame you guys. Then they make the musical atrocity that is St. Anger. So they all apologize and say no, no, no, we'll do it this time. Then they release Death Magnetic, which really is their best album since ...and justice for all, I'll give them that. However the production is bad. Also, it sounds like Metallica trying to play 80s metallica, and kind of pulling it off, not Metallica just being Metallica and sounding good, because they aren't that band anymore. Thats just how I feel.
I don't think age really has anything to do with it. There are many many thrash bands that didn't enjoy the success that metallica had in the 80s but are still around, releasing albums (Kreator, Sodom, Slayer, Megadeth) that are on par with their early work. Personally, I think the only people who are really still interested in playing metal in metallica are James Hetfield and maybe Rob Trujillo. I think that because of interviews I've read with Hammet and where he wants to go with his music or whatever. As for Lars, I think that because he's somehow gotten worse since the AJFA. So I think he doesn't practice or something.