So if you don't buy health insurance, you are increasing costs for other people. The federal government is taxing you to recoup some of those costs. An analogy would be taxes on alcohol or tobacco, although these taxes are usually worked into the retail price of the goods so that people don't even have the opportunity to refuse to pay them. Another example would be taxes on an enterprise that is creating additional costs to the environment through pollution; the government taxes you if you don't purchase and install anti-pollution equipment. If people don't purchase the pollution-control equipment and won't pay the tax, the government will fine them too.
Again, you may object: isn't the argument that you are imposing costs on others really an argument that young and healthy people are subsidizing other people? Why should I be forced to pay taxes that benefit others more than they benefit me? The answer is that tax policy does this all the time. Progressive income taxes, for example. tax subsidize middle class and poor people at the expense of rich people. You can't get out of paying taxes just because your tax dollars subsidize people you'd rather not subsidize. This is as true of rich people who object to subsidizing the poor and middle class as it is of people who have ideological objections to government health care programs.