It is true in some cases, but is not a universal truth.

Jeri

Not at all. It seems to be mostly a phenomenon of drier climates where a more developed root system leads directly to a healthier plant. I see that in potted plants often, but once they are established in the ground, WYSIWYG.

Rules of thumb across all species of everything: babies are more vulnerable.

And there are also sickly babies and healthy, bouncing babies--at least according to my Mom (comparing me and my sister). With some TLC and luck, even sickly babies can outgrow their sickliness and become healthy adults--though my sister (a sickly babe) claims she is still more susceptible to some diseases--but I claim she has just been conditioned by mom's stories of sickly/healthy babies into believing her general overall condition still is more sickly than mine.

Don't know if a rose can be trained to think of itself as sickly, however--but I have noticed that some of my weaker beginners became stout and healthy after a couple years of care. But I did have one rose that just never got over being weak and wimpish no matter how long I encouraged it.

Kate

I have found that most of my roses do better the longer I have them (no spray, except for certain circumstances like a bad spring for canker, etc).

It could be cultural. I garden on old land right by a forest and use forest soil to some extent in all my plantings, because my grandmother did. I have amazing biodiversity here, too.

So I don't know if it's maturity of the rose or just a different way of treating them, but I'm fairly sure I'm not just bonkers :)

What Madgallica and Jeri said.

I haven't seen it at all in any of my roses.

I wish that were the case!

When folks post often, it is difficult to find a particular thread without the correct key word or phrase. henry_kuska has posted something documenting how this effect can be related to establishment in a healthy environment - healthy in terms of the balance of the soil's organics/organisms, but I can't find it right now.
I've observed it with most of my older roses. I also discriminately buy very disease resistant roses so the causality is not completely clear except that they aren't so resistant while they're getting established (are young) which here is about the first three years.