According to the author of the book, Barzillai was an old man who was 80 years old and helped the king by giving him food and supplies because he was a very important and kind person.
2 Samuel 19:32: Now Barzillai was a very aged man, [even] fourscore years old: and he had provided the king of sustenance while he lay at Mahanaim; for he [was] a very great man.
This verse states that David died at an old age full of days, which contrasts with the depiction of age and health in 2 Samuel 19:32.
1 Chronicles 29:28: And he died in a good old age, full of days, riches, and honour: and Solomon his son reigned in his stead.