A health insurance program is, necessarily, a program to transfer resources from the less-sick to the more-sick. Since people tend to develop more health care problems as they age, that means the existence and widespread provision of health insurance is something that older people are the primary beneficiaries of. In that sense, Ben Smith is quite right that Obamacare will likely be a poor financial deal for the younger cohort that voted loyally for Barack Obama's election and re-election. (For a less polemical look at Obamacare's effect on young people, read Jeffrey Young's story here.)
The primary flaw in Smith's analysis, however, is that obviously today's young people will be tomorrow's old people. So even if the existing under-35 cohort has relatively few health care problems right now, it's still in our interests to set up a system that will provide for future needs.
The converse, however, isn't the case. The GOP's base of senior citizens can afford to be genuinely indifferent to the plight of younger people who do have substantial health care needs because old people don't get younger. Nor do today's elderly cohort have that much cause to concern themselves with the question of the long-term sustainability of Medicare, the integration of information technology into the health care sector, the potential public health benefits of restaurants disclosing more nutritional information, or most of the other subjects the Affordable Care Act tackles. There are also side features—like the regulation allowing kids in their early twenties to stay on their parents' health insurance policy—that are very favorable to young people.
But honestly, the issue here isn't in the details. It's in the fact that today's 25-year-old is tomorrow's 55-year-old. Whether the Affordable Care Act is really in the interest of the young cohort just comes down to whether or not it's a good idea overall.
The place where raw conflict of interest comes into play is that the Affordable Care Act is a fairly massive income redistribution scheme. The tax base to finance Medicaid expansion and exchange subsidies is extremely progressive, not flat like Social Security. The richest Americans will overwhelmingly be the ones paying the tab, and the benefits accrue overwhelmingly to the bottom half of the income distribution.