Yeah. there is some obvious logic that one can use here without having to look at data.

Not everyone survives to write to an old age.

Old people have health problems that can prevent them from work, like going blind.

People who write a great work at an old age will not have the time and energy to do all the non-writing parts of making the great work seen by readers - which has always been a big part of writing. Like getting their book in bookstores, advertising it, etc.

If someone is a very talented writer they are likely to write great stuff before they get old and may spend their old age preening and working on their legacy instead of new works. They will already know they're a great writer, so the drive to make another great work is lessened.

If someone is already an accomplished writer more of their time will be taken up with invitations to speak, being on award panels, doing interviews, writing introductions.

There is less financial incentive to write a great work when you're very old.

It is harder to be part of a literary salon full of smart people that help you grow your creativity when you're very old.

As people grow older they become more alienated from the zeitgeist and are better at connecting with their own generation.