How do Catholics explain that the fighting leaders of the Hebrews were old men? In the books of Joshua and Judges the leaders of the Hebrews are old men like Joshua himself or Judah. In particular the following verse made me wonder something:

Judges 1:3 (ESV):


  And Judah said to Simeon his brother, “Come up with me into the territory allotted to me, that we may fight against the Canaanites. And I likewise will go with you into the territory allotted to you.” So Simeon went with him.


When I first read this verse I understood that Judah himself went to battle against the Canaanites, but he should have been very old at this time. Then I realized that Judah was also very old when leading the Hebrews. 

The Hebrews battled against a lot of countries at this time and I had always thought that the leaders engaged themselves in battle. Now I want to try to understand this and looking at different versions of the bible I found that the same verse of the NIV version is very different:


  The men of Judah then said to the Simeonites their fellow Israelites, "Come up with us into the territory allotted to us, to fight against the Canaanites. We in turn will go with you into yours." So the Simeonites went with them.


Reading this, I understand that Judah and Simeon didn't fight but instead the (presumably younger) members of their tribes were the ones who battled against the Canaanites.

How do Catholic Christians explain this? Did the older men fight themselves or were they just the leaders of the Hebrews, but didn't engage in battle?
I don't have a specifically Catholic perspective but by the time of the events in the book of Joshua, the physical people Judah, Simeon, and the other 10 patriarchs that the tribes of Israel were named after were long dead. 

Here's what's going on:

The 12 tribes of Israel were named after the sons of Jacob (their births in Genesis 30). One of those sons, Joseph, ended up in a high position in Egypt despite a lot of extremely unfortunate events and invites his family to live with him to save them from drought and live prosperously in Egypt (Genesis 46). Generations pass (meaning all those patriarchs have died), a new pharaoh takes over who hates the Israelites, and the stage is set for Moses's story (Exodus 1). Exodus 1:6 specifically says that Joseph and all his brothers had died.

By the time of the events in Joshua, Moses has already led these people out slavery in Egypt, is dead, and the descendants of the 12 sons of Jacob have inhabited the land God promised them - more or less (lots of wrestling over the land is happening at this point). The region is eventually won by the Israelites and divided into 12 territories, one for each tribe that traces its lineage back to a son of Jacob (Joshua 14).

So...

Long story short, by the events you're referencing in Judges 1, Judah and Simeon have been dead for a long time. The name "Judah" at this point refers to the descendants of Judah, and "Simeon" refers to the descendants of Simeon. Referring to the group of people by the name of their well-known ancestor was common at the time.