Tolkien characterized Balin in the Hobbit as "very old-looking":
... Instead there was a very old-looking dwarf on the step with a white beard and a scarlet hood; and he too hopped inside as soon as the door was open, just as if he had been invited. “I see they have begun to arrive already,” he said when he caught sight of Dwalin’s green hood hanging up. He hung his red one next to it, and “Balin at your service!” he said with his hand on his breast.
The Hobbit Chapter 1
Readers also found him to be wiser than Thorin:
Balin is portrayed to be the wiser, more social and skeptical individual in Thorin and Company. He is kind, sly and brave, an excellent fighter. Thorin, at one point, quoted on Balin's skill as a fighter. He develops an essentially strong relationship with the other Dwarves he travelled with and was willing to fight alongside them for any cause. He was sympathetic, sometimes cynical, towards Bilbo Baggins, remarking that the latter wasn't fit for the job they had set out to do. He got on brilliantly with Thorin, and seemed to share something of a grandfatherly view of the latter.
LOTR Fandom page on Balin
Understandably, Peter Jackson's 2012 movie The Hobbit: An Unexpected Journey portrays Balin with his hair completely white and looking decades older than Thorin while the 2001 book The Hobbit - An Illustrated Edition of the Fantasy Classic illustrated by David Wenzel portrays Balin also looking older than Thorin, though not as exaggerated as in the movie.
From Peter Jackson's movie:
From David Wenzel's illustrations:
But according to the genealogy in LOTR Appendix A - III (Durin's Folk), Tolkien had Thorin to be born in TA 2746 while Balin to be born in TA 2763, making Thorin to be 17 years older than Balin. In the scene, Thorin would have been 195 years old while Balin would have been 178, both were in the same age bracket considering the usual lifespan of dwarves is 250.
The question: Why does Tolkien make Thorin to be older and contemporaneous with Balin, only to present Balin in the Hobbit as "very old-looking"?