My take on opinion-based questions is the following:

A question is "primarily opinion-based" if every reasonable answer would be equally valid.

There are quite a few problems with opinion-based questions, especially in the context of magic. For a longer discussion about my stance on the small line/ the big blurry region see my answer to The problem with opinion based questions. But for the sake of this question let's examine the most important parts of the above statement.

Let's take a basic example to evaluate the different parts and assume we are talking about a question like:

What could be elements I could add to my magic?

The first part of my analysis for "primarily opinion-based" is to realize that there might be reasonable answers and there might be answers that are unreasonable. Unreasonable answers shouldn't be taken into consideration. An example of an unreasonable answer would be:

Fries and Ketchup!!!

While I am getting hungry reading this it certainly is not a reasonable answer to the proposed question. It's important to realize that this is already a part of my big blurry region - there might be some people who would say that this answer could be valid if it was supported by reasonable explanation.

This is important because the next part of my analysis is whether all answers would be exactly equally valid.

Metal, Wood, Fire, Water, Air - these are typical things that are attributed to magic systems. They are reasonable and they could be supported by citing some references or simply showing that others did that, too.

But what about Light? Or Darkness? Or Void? These seem valid, too. And currently each of these 8 suggestions that I offered would be equally valid. There are no criteria why one of them would be better than the other.

It's not a problem that they are all valid - the problem is that they are equally valid.

The question could be salvaged by giving more information - what is currently existing, what aspects are missing, what was the inspiration, ...? If there is an element for change or strenght missing, possibly related to the colour "red" and inspired by chinese mythology you could very well support that Fire is the most appropriate answer. While other answers could be supported to be correct by redefining some properties, the existing example is simply better. Now we have a hierarchy - there are many valid answers, but one is clearly better than others. This is of course just a very simplified example.

This aligns with your view that a magic questions where answers can't be supported by logic or facts are primarily opinion-based - after all the answers can only state their opinion without any more reference than "I think it would be cool".

We might still disagree on certain implementations of these rules - as I've said, some people might think that the criteria I offered to safe my example question are unreasonably strict or that they are still not enough because there could be multiple answers that are better. There will be disagreement from time to time, but that's what community voting is for.

In my opinion magic is a bit opinion-based, but questions that are not trying to provide any criteria for why certain answers would be better in their specific context should be put on hold.

The specific example you referenced mentions enough parts that make me say that it's not opinion-based - the OP is searching for reality-based information about rapid aging and reasonable problems that might occur. There are many possible answers that are valid, as can be seen by the amount of "This will hurt" and "This will not hurt" answers, but each reasonable one is supported by different science-related facts that can be evaluated.