I agree that on average, this is wrong. But in this case, I’m talking about certain instances which, while they are not happening most of the time, still exist and exist enough for them to impact nearly all of us in some way, shape, or form at one time or another in our lives. I agree with you that overall, people become more responsible as they get older (hence the examples you used). What I was pointing out was that while this is the general way we trend as human beings, it is not a given for all people all of the time.
As for whether or not we would find that repaying debt indicates that a person is more likely to act in a responsible and caring way when it comes to their personal relationships (for example, are people who make their credit card payments every month less likely to be unfaithful to their spouse?), I have no idea. But I’d definitely be interested in the results of a study that examined the relationship between those two variables.