This is a legal requirement for all Telcos, not just Telstra. Optus pre-paid deals are pretty good and i believe you cn even set up an auto monthly top up option so its essentially just like a plan. But if you had one in particular you like then you just need to find an 18+ year old who trusts you and they can open one for you. You just pay the bills and use the account etc.
They would need to fully understand they would be the ones liable for any payments if for some reason you couldn't pay. This includes standard costs as well as any unusual costs (like excess data fees, late payment fees etc.) They can easily transfer the account to your name when you turn 18 too.