Yes, the individual mandate is necessary for risk pools - not just for the old but those with pre-existing conditions and the unhealthy. It is irresponsible if you don't have insurance. Your interpretation is simplistic and inaccurate. Young people do not "pay" for the old and sick, they pay for insurance. The "old and sick" have Medicare.  I am healthier, have a better diet and more active, as an life long athlete at 61, than most young people.  To equate young and healthy is completely inaccurate in this country.
You know obesity is an epidemic, right? Rates of colon cancer is rising fastest in young people (30's). They get sick. There is an attitude among young people that they don't need health care, when they do. It costs taxpayers when people go to the emergency room and/or  are unable to pay for the care they receive. It was this attitude I was addressing. The ACA does need to be fixed.  Republicans did undermine some key provisions in the law (before they voted against it).