I think most artists or musicians career peak occurs between in the mid to late twenties - this holds true with Metallica and a lot of other great artists - the most revered usually being the ones who managed to die before age blunted their creative talents. I think the main reason for this is that this is usually the age you've mastered your instrument but still have enough youthful exuberance to truly capture the spirit of what you originally set out to do - some artists can see a mild creative renaissance after this period in their lives but it's few or far between - at this stage I think its fair to say Metallica are extremely unlikely to release another classic, great or even good piece of work again - even with a good producer/engineer.