You get older.
As you get older, you have more to write about. That's just an objective fact: you've had more experiences, and you've developed the wisdom to reflect on those experiences. And you've had more time to read the works of the great masters and to perfect your own technique, learning from your youthful mistakes.
Naturally, then, your writing should get better over time. The wellsprings of creativity should nurture crops that yield an ever more abundant, ever more delectable harvest. As the years pass, the masterworks should fairly flow from your brain; and by the time you reach old age, the floor should be littered with a constant blizzard of sheets filled with brilliant prose, piling up like toilet paper dispensed by an exuberant kitten.
So, why doesn't it always work this way?
This is the part that's unsettling to think about. The thing is, when you're younger, you have a certain bravado - a certain chutzpah - that's very difficult to recapture when you're older. With age, you become less spontaneous and more critical. You see problems more easily than solutions. You become preoccupied with technique. You second-guess yourself.
It's hard to think about this because, obviously, we are always getting older. And we like to think that our suffering has meaning. We like to think that the dues we've paid, the bitter lessons we've learned along the way, have not been for nought, but have given us hard-won grist for the mill; we don't like to think that the mill itself may be getting worn and creaky.
So, that's the problem. Now I don't mean to suggest that the process is inexorable or inevitable. There's still the positive side of the process, too, the benefit of experience. As to which one prevails, I think it depends on the individual and on the style of writing. For a Romantic poet, age isn't an asset, as this memorable review of an edition of Wordsworth suggests.
What's the solution? Right now, I don't have any good answers; but I am wrestling with it.
Which is another way of saying, I'm back to writing.