For instance, comparing a new A320 to a 20 years-old A320, how can I tell people that a new A320 is more reliable by not just saying because it is new?
That is assuming a brand new one is actually the most reliable.
Not sure how well it applies in 'real life' to aircraft but there is the well known 'bathtub' reliability curve which shows higher initial failure rates due to infant mortality followed by a long period of low failures increasing at end of life/wear out.
I suspect that if noticeable at all the infant mortality period would only be a few hours/cycles.
Anybody here have any practical experience or data ?
The other point to make is that the brand new one is not exactly the same as the 20 yr old, reliability should be better due to incorporating lessons learned over the years.