I have never heard of any rule saying people CANT fast.
Also I believe abstinence is binding for your lifetime unless specific medical exemption exists. I think it's only fasting that you're exempt over 60
Right. It's important, however, not to be scrupulous about not fasting when Church law indicates you don't have to. Now, one of the biggest mortifications of fasting is the mortification of the will, through obedience. I won't eat meat today because the Church forbids it. I know a lot of people who agree that they can go a day or two without meat and not even notice it ... just because, say, they had fish for dinner, etc. But it only becomes "hard" when you are not allowed do it. It's because the will is what needs to be mortified the most.
So, if there were a law that actually stated that you CAN'T fast (and there isn't one of course), then it would be a mortification not to fast when you might actually want to.
There's a story about St. Francis of Assisi where he had gone a number of years without eating meat. At one point, he and some of his religious brothers were guests for a meal at someone's home, and St. Francis ate the meat that was served without giving it a second thought. So one of his brothers asked him about how hard it must have been to eat the meal and thus basically lose his fasting streak. He responded, not at all, since charity toward his host was more important than his fasting. So here was a man who was not attached even to the fasting. In some stages of the spiritual life, people can get attached to spiritual goods, such as fasting, and even that attachment is not compatible with total sanctity.