For some bizarre reason there is an obsession in the media about Obamacare needing young healthy people to sign up for the program to work (e.g. see Ezra Klein today). Actually, the program needs healthy people to sign up regardless of their age.
The logic is that healthy people who get little care are in effect subsidizing the care of the less healthy. This is every bit as much true for older healthy people as it is for younger ones. In fact, the subsidies are considerably larger in the case of older healthy people since people in the 55 to 64 age group will pay roughly three times as much for their insurance as people in the youngest age groups.
The likelihood of someone being in good health gets smaller as they age, but there are still millions of people in this older age group who will use very little health care over the course of a year. Their money will help support the program every bit as much as the money of younger people.