Shusha, you make some good points, I think you are erring a little far on the side of minimal changes to the musculature from aging...but that's ok, I get your point.
Okinawan Karate has some long lived folks, guys that are in their 70's that are still quite healthy and mobile.
However...TKD kicks are a different deal, the level of flexibility required for TKD kicks is WAY beyond the normal range of motion for human beings old or young. So sure, you might be able to get away with it for a long time but two things are sure to me:
It WILL get harder for you to maintain that kind of flexibility and be injury free as you age. For all the TKD guys I've known personally, they seem to give up on the head kicking so much in their 40's and beyond. I'm sure there are extraordinary individuals who make it work, but then there are probably guys who kick my ass with Capoeira too, says nothing about the practicality of the art or training.
For someone deciding on an art, it's worth asking whether they are willing to spend alot of time on the conditioning to acheive kicking like that, when there are styles out there that don't require so much work on being hyperflexible.
Not trying to put down TKD, just how I see it in terms of the original post.
barring injury or disease, there are many older martial artists who retain their range of motion.
Absolutely, but in most cases this is normal human ranges of motion
which as I said, much of TKD kicking goes way beyond. Joints in old age have a hard time with things like high side kicks..I have personally known a number of people who have injured knees and hips by repeated high yoko geri...
That includes developing your flexibility which is absolutely proven to improve with training, whether at age 20, 40 or 70.
Again yes, within reason. Modern TKD kicking is well outside that range, and having a 70 year old try to kick like that would be ridiculous, as well as risky.
This leads many who study aging to believe that loss of flexibility with age is a function of disuse rather than physical changes.
That is not correct, it is agreed upon by most of the medical community that elasticity of tendons, ligaments, mobility of joints decreases with age. That is not a command to not do anything, nor a claim that we are all doomed to inactivity.. just food for thought to be rational about training, and frankly having a 60 year old do TKD (at least TKD with ballistic high kicking) would be a very irresponsible recommendation.
My point to the original poster was not to limit himself before he even began a training program or martial art. He shouldn't walk into a school and be thinking, "I'll never be able to do that." That is what training is FOR. To teach you to do that. A good training program will give you the tools you need to perform the martial art according to its standards. That includes developing your flexibility which is absolutely proven to improve with training, whether at age 20, 40 or 70.
There are all kinds of schools out there teaching stuff that is insane, and terrible for you in terms of exercise. What is healthy and ok for a 40 year old may not be for a 60 year old etc. I don't think he should limit himself, but he is right to ask whether something fits him or not, and not just assume that because it works for the people in class, it will work for him.