True Martial Art? That is a tough question and I can understand why Terry had no answer. Is a true martial art just a method of defending oneself, many modern practitioners think so. But to a Chinese or Japanese practitioner from 100 or more years ago that would only be half of a true martial art. And yet to an ancient Greek a martial art was a very pragmatic method for killing enemies and nothing more.
A more cynical modern take would be to say a true martial art is one with history and substance, some might even use such a view to say TKD is not a true martial art because it doesn't have enough history, but that's not right.
The funny thing is all of us with some little experience can look at a system and tell if it is a true art or not, but we find it very difficult to express why we can do this.
There is something in the systems, the methods of teaching, the approach of the teacher, and the experience of the founder which gives us the insight into the genuineness of the art. But ask us to tell you what that is and we cannot give an answer.
Yes, exactly; we can see something, some specific set of qualities, in what it is that we practice that we think makes it a true MA, and in what others practice that we would describe in those terms. The point is, we can see it
knowing nothing about the moral biography of the practitioners. I think it's a category error to conflate the system with the individual who practices the system; what Terry is asking about is, basically, what is there that makes a certain body of knowledge a true martial art, as opposed to... something else.
And just as you can't say that one MA is a better fighting system than another—isn't one of the standard conclusions that emerges from such threads that it's the fighters, not the systems, that are better or worse than others?—you can't say that the moral perspectives or ethical values of the practitioner make the system s/he is carrying out a MA or not. But as ST says, we can look at a system, as implemented by an experienced exponent of that system, and tell if if it's an MA or not,
even though we know nothing about the person or people we're seeing practicing that system. That's the crucial point, I think, and also the crucial question:
what is it that we're seeing that says, this is a true MA?
I don't think it's 'venerability'. How old is karate, for example—a century, a century and a half? How old is Krav Maga? How old is Hapkido? And where is the book in which the minimum age of a fighting system to be a true martial art is written down, come to think of it?
My guess is that a true martial art has three necessary properties: (i) it's systematic (its component skills connect to each other consistently), (ii) it's comprehensive (it covers a very large range of self-defense situations), and (iii) it's effective (it leaves you standing and the other guy... not so much :EG

. There's probably more, but those three criteria are I think the sine qua non of a true martial art... in the sense that we wouldn't describe any system that lacks them as a true MA....