When it comes to Qantas, I find it’s less about age and more about era and base (if that makes sense).
Qantas have numerous staff on numerous different employment contracts in numerous bases. Each one has its positives and negatives.
Overall I’d say the old contract, Sydney based staff are the worst Qantas have (overall I mean). If I’m getting on a 747 to Sydney I always prepare for the worst. There are some good ones, but virtually all of the Qantas shockers I’ve had over the years are on long haul services with Sydney based staff.
The Brisbane base is very senior, and I used to enjoy the 747 BNE-SIN flights as the crew were usually very good.
There’s then the mix of domestic/NZ/UK/380 etc crew who are usually younger, and can be variable, but generally not awful.
If you’re routinely flying SYD-SCL, I can see why you’d be experiencing the worst of them