As long as I live, I will never understand the “they’re going to do it anyway so why should we try to even pass rules concerning their health” argument.
If it’s good for their health to raise the age limit, it’s good (and I believe it is). What they do “anyway”, why does that matter?
Why do you believe it's good for skaters' health to raise the age limit (and only for seniors), though?
I'm going to try to take a stab at this from what I believe is to be a non-biased view:
How do we know training quads is dangerous? We can talk about theory - like growth and the formation of bones, but all studies are conducted through experiments and analysis. To my knowledge, none of these studies have been published from a reliable source. No one has compiled a comprehensive study from the data we currently have. For something like this, we can only infer a correlation, not a causation. To do this, we'd have to look at the available data and break it town into three separate variables:
1) Does training quads lead to more injuries than not training quads (training doubles, triples, spins, skating skills, etc.): I don't have enough knowledge to answer this, it'd actually be very interesting if someone has compiled the data from current and past skaters.
2) Does training quads pre-puberty lead to more injuries than training them post-puberty: for the ladies we cannot say as there is no data on this currently, for men we have to look at skaters who trained quads pre-puberty and those to trained them only after puberty - again I do not know myself.
3) Does increasing the age limit prevent skaters from training quads (or is the answer that we have to apply a minimum age to start skating as a whole): This is the only question I can answer from the data I've seen in the ladies, and that is no. Quads are not trained for the sole purpose of senior competitions. In fact, all the ladies who have ever landed a quad in competition did it first in juniors, with the only exception of Tursynbaeva. Many of these ladies have started training these elements way before they even made their international debut (Shcherbakova, Liu, unsure about Trusova and Tusynbaeva, and Kihira and Tuktamysheva's 3A). So to reduce the quads, it would make sense to raise the age limit for juniors, ie. all international competition, to 18, or whatever age threshold is believed to lower the dangers of training quads. This is only the incidence of quads though, nothing about health (that is answered by the first two).
4) Alternatively we can also look at if increasing the age limit reduces injuries overall by looking at percentage of injuries in skaters above 18 and below 18 (if that's the chosen age limit). But this only tells is about figure skating as a whole, not about the quads and this would be about all competitions, not just seniors.
I think those 3 questions have to be answered before we can say that a rule to increase the age limit will be healthier for skaters, or if it even affects health at all.
Obviously, none of the studies would have the appropriate control: one skater cannot be used to test all these things so the biological and environmental variables would not be accounted for. Then, if we were to compile all the data and conclude that an increase in age would be healthier, what makes 18 the magic number? Why not 17 or 20? We'd need the study to determine the age.
This age limit argument has been going on for so long with both sides arguing for what they want for whatever reasons, but I don't think there has been any logical approach to answering how age limits affect the health of skaters.