The sentence in the title has two, almost contradicting interpretations. (1) that you are not obligated to do the task "work until you're 30", or (2) you are not obligated to do the task "work" until after you turn 30. What's the easiest way to clear up this ambiguity?
Without any more context, I suspect most people would interpret it as meaning (2). I did, and had to think for a few seconds until I saw the ambiguity to which you refer! You could remove the ambiguity by rephrasing in various ways. Here is one:
Your obligation to work begins at age 30.
although if you want to retain the specifically negative aspect of the original, the following would be better:
Your obligation to work does not begin until age 30.
If meaning (1) was actually what was required, then one way of saying that is:
Working until age 30 is not an obligation.