Keep in mind that as many of these performers get older their life focus changes, as it does with all of us. Often this brings about a more reflective tone to their songwritting, more serious, and often a bit laid back. At a certain point in life we turn our focus to what we have done, what could have been different, where we are going from here, and all the lessons life has taught us along the way. So of course you'd expect that rather than full of energy about to take on the world as they were in their youth, their music would be of a different nature. This is a natural part of aging, maturing, and hopefully acquiring some wisdom. Life's journey revealed to us in music by those awakening to it's changes.
I could have stated this better the first time around.