How to interpret "Jim Scarborough'd never carried one; that's the younger Jim." in No Country for Old Men

Solution 1:

It's rare that we get a chance to provide such a definitive answer here at EL&U... but the translation is WRONG! and your first impression was right

But it's understandable why they made that mistake. The casual, wandering, way of adding to what was just said reflects a way of talking that is very realistic for that character. But it's "colloquial" in form and relies entirely on context, so it's not usage that would appear in an English textbook.

p.s. The way that the character might have expressed the second meaning {"when he was young."} would be more like this: "Jim Scarborough'd never carried one; until he got old." or "Jim Scarborough'd never carried one; at least while he was young."

Solution 2:

There apparently were three Jim Scarboroughs; three generations; all sheriffs of Kleberg county. This would lead me to believe that the second Jim was being mentioned. From the book,

The Texas Sheriff: Lord of the County Line by Thad Sitton

...all Kleberg County Sheriffs had the same name--father, son, and grandson--Jim Scarborough.