Have you ever noticed that when you WANT to be a runner, lots of people who don't want to achieve that are always ready to try to talk you out of what you want to accomplish? What's behind that anyway?
Some are already seasoned runners who just don't want you to be a runner as well. It's like running is their little elitist club that they don't think you should be allowed to join, as if you even needed their permission. Hmm. (Be successful just to spite them.)
But I think that many of the naysayers don't want you to run because they don't want to put in the hard work over the long time that it will take to shape you into being a runner. They don't want you to accomplish something for yourself because they would want instant results to become a runner as well. It's better to hope for someone else to fail than to let them step ahead of you in something you don't even want for yourself? Wow.
Running is not bad for someone my age, per se. It's running without consideration for the sedentary middle aged person's current level of fitness that is the problem. Diabetics have considerations. Overweight / obese people have considerations. Weak people have considerations. People who drink, smoke, eat poorly... Yep, even more considerations.
I'll be honest with you -- when I first started trying to jog a few years ago, all the ligaments in my ankles crackled painfully. It was like my ankles were made of glass and every step was close to shattering them. My guess is that all my leg and foot muscles necessary to support my ankles were too weak to deal with the extra force of running under my weight. Enter strength training. The crackling was not there when I ran my first 5k in 2011.
I started really training for running in May 2012. Shin splints have been the dominant problem, again, likely because other muscles needed to be strengthened. I really thought all this time that I would be predisposed to knee problems. Nope.
I believe that if you are already a lightweight, fit, young person, taking up running will be simple for you. Add years, and you will need to add some caution. Add pounds, add some more caution. Add health conditions, add more caution. Add years of sitting on your lazy butt, and you will add more cautions, no matter how thin you are, because your cardiovascular system will need to strengthen. Try to jump in whole hog with an intense program and you will hit a brick wall. You will get hurt. You may cause yourself cardiac problems.
My advice? If you are in your twenties, it'll be a lot easier to get into great shape and STAY THERE if you do it now. If you are in your thirties, it'll be a little harder, but still very worth the effort.
If you wait until your forties, when you are overweight and starting to have health concerns? It'll be a lot harder to get healthy again. You should still do it. :). I think your quality of life will improve tremendously by getting fit, no matter what your age is. Go for it!
By the way, I'm in running for the memory of the easy speed I used to have as a kid when I was physically much more streamlined. I want to feel the wind in my hair once again. Nothing wrong with that. :)