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Ok - I practice in Tennessee.
In paternity cases and voluntary acknowledgment of cases, retroactive support may be ordered from birth. The court may also award child support retroactively to the date of separation of a married couple, abandonment of a child, or physical custody of a parent or non-parent caretaker. Retroactive support is calculated using the average income of the parties over the previous two years. Other provisions may apply if a parent has intentionally hidden the child to prevent visitation.
If the court has entered an order stating that she waived back support, she should be estopped from coming back and claiming a need or entitlement to this money. This is called res judicata.
Also, the judge will clearly see that she's trying to get the money for her own use as he will be an adult in a year (eliminating your obligation to pay).
If there is a court order already determining this issue, it should be over for all purposes.