puts him at age 65. Sounds like a reasonable age for him to still be able to write books.
John Grisham is 62 and he's published 8 books in the last 5 years. Stephen King is 69 and he's put out 3 books the last 4 years.
I think Sanderson is more in their ballpark of writing frequency than he is to Robert Jordan, George R.R. Martin and Patrick Rothfuss who struggle(d) to put out books in a timely fashion.