I think the urgency of creativity in a young musician can slowly ebb away as they get older, but I think one of the reasons why they produce less immediate work is that they simply don't have to try as hard, because they have success and money, and a degree of acceptance of what ever they deliver.
Rick Davies of Supertramp said that he had been dreading the reaction to 'Breakfast In America' which he felt was nowhere near as accomplished as 'Crime Of The Century', an album he thought they would never match. He admitted in interview that he had simply stopped trying as hard to write really great songs, and that his quality control was nothing like as rigourous. When BIM became their biggest selling album by a long way, he realised that a reputation goes a long way, and the impetus to try and top a great album is simply not important.
Of course, that doesn't apply to every artist by any means - people like Bruce Springsteen have consistently made great records, even if without the immediacy of 'Born To Run' and 'Born In The USA'.
I talked to Mike Rutherford about it, and he said that he gets annoyed when people ask him why he still writes, records and performs, even with his millions in the bank. He said that the inference is that he only made music to become a millionaire, and now he is one, he should give it up. As he said, "I am a musician, this is what I do. What else whould I do? It's all I'm good at..."
That, I guess is your answer.