Yea, I can't think of any reason why turning 18 releases you from this obligation. If it was the case then everyone would simply not pay and wait until they turn 18
If this were an amount due under contract, being a minor at the time of it's formation might have made a difference, but since the amount is due for a fine, it's a different story.
Depending on how large the amount at stake is, you might check with a local attorney as to the effect of the state referring the matter to collections. If the state has written the matter off you may be able to settle for less with the collection agency.
Contracts between private parties might have a different result. But you are dealing the government enforcing actions on public roads and your privilege to driver (not a right) and fines for violations of law. No, you cannot ignore your fines.
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