Let's do some math on the ages
Okay, from Rooster's tombstone, he died in 1903. Their adventure takes place 25 years earlier, so that's 1878. The tombstone says he was born in 1825, making him 53 at that time. Hardly an old man, but old enough for those times, I would say.
But at the end, Mattie says that she's like to see LaBoeuf again, and that she figures he would be between 70 and 80, closer to 80.
That doesn't make much sense. Let's say he's right in between, 75. That would place his birth year as being 1828 - only 3 years younger than Cogburn! And Cogburn was an old man even in 1878! How could there be that much difference between the two of them in terms of their physical appearance and age, but only 3 years chronological difference?
I want the doctor to take your picture so I can look at you from inside as well.