As a minor, your parents decided for you, but once you're 18 nothing is stopping you from either trying to re-negotiate the contract (i.e. become politically active) or trying to find a better deal (emigrate). You can also decide to simply unilaterally break the contract (become a criminal) and in practice you could even live without such a contract in a variety of vast areas that are largely uninhabited.

You're not being compelled to accept this contract, but it's such a good deal that few people decide otherwise.