National seems to typically have the newest fleet. I've heard from several people, including National employees that as soon as vehicles hit a certain age/mileage, they punt them off to their "leisure" branded companies in the same ownership group: Alamo & Enterprise. I've heard Avis also does this with Budget, but don't ever use Budget so I can't confirm this to be true.

I'll also say that the few times I've had to use Hertz in the past ~10 years, the vehicles seem way older than National or Avis cars. However, that's <10 rentals in only a few locations, so I can't make a broad statement like "Hertz has older cars than XYZ" based on my limited experience with them.