Mt gut feel after flying multiple types is that 121 aircraft don't really get "old" until about 20 years.

Fleets that are older than that just seem to break more.

Pretty consistent across all mfg, they're all designed to the same regulatory standards.

Pax carriers, especially recently have opted to replace fleets before they're really old due to the availability of more fuel-efficient models. That's driven by carbon and 21st century gas prices.

Cargo planes typically fly less hours, so they don't burn as much fuel in a given month. So they tend to prefer the lower mortage/lease rates of used planes, since the fuel savings of new designs only apply if you actually burn gthe fuel.