An NYT article on the winners and losers from the bill listed among the losers people who buy individual insurance, since it will leave insurers "stuck with more people who are older and ailing." The issue here is ailing, not older. The exchanges can already charge different prices based on their age. While the law limits the band between age groups, so it's not exactly equal to the difference in costs, this is a relatively small matter. The health of the people within an age group makes far more difference.
It is also important to note that many of the people who are predicted to go uninsured because of the repeal of the mandate are people who would have otherwise gotten Medicaid. These are people who would effectively get free insurance if they applied on the exchanges but won't make the effort without the mandate.