A rule? Nope, but think about it. Would you have been able to do what you did without them? Would they have been able to do more for themselves if they hadn't given to you?
We used to take care of our family, our parents, our elders. Now the trend is to stick them in some hole in the wall and forget about them. Those in the 50 to 65 age range now were the ones hardest hit by the 01 and the 08. Millions lost everything, retirement, homes, jobs, ect... All they had was diverted to their kids, who tend to live at home and use those limited resources until a much older age now. We are facing an entire generation that will be retiring over the next two decades, that have nothing to retire on.