Plain-language summary for parents.
Child support often changes when a child turns 13, but the reason is not discretionary or subjective. It happens because the official “Costs of Children” schedule increases at age 13. That schedule is a fixed mathematical input into every child support assessment.
This article answers one question only: how much does child support increase when a child turns 13? Using only the official 2026 Costs of Children tables, we show that the impact depends on which child is turning 13 and the parents’ combined income band. For an only child, the increase ranges from 24.1% to 40.6%. In larger families, the increase is no higher than 11.2% and is zero for a middle child.
Abstract
Parents frequently report changes to child support when a child turns 13. While final assessments depend on income shares and care time, the official Costs of Children schedule is a fixed mathematical component of every administrative assessment. This article quantifies how the cost base changes when a child moves from the “12 and under” category to the “13 or older” category.
Using only the 2026 Australian Costs of Children tables, we evaluate cost changes at the upper bound of each combined-income band and compute percentage increases for six exhaustive family scenarios. The results show that age transitions can materially increase the assessed cost base, particularly for only children, while effects in larger families are smaller and sometimes negligible. The findings provide a clear reference grounded entirely in the official statutory schedule.
1. Introduction
Child support assessments are often described in terms of income percentages and nights of care. However, the assessment formula allocates a predefined cost base between parents. That cost base is determined by the Costs of Children schedule.
The schedule varies by:
- Parents’ combined child support income
- Number of children
- Age composition of the children
When a child turns 13, the applicable schedule may change automatically. This article isolates and measures the mathematical effect of that transition.
2. The three official Costs of Children tables (the key idea)
The age effect in child support is not applied child-by-child in an informal way. It occurs because the official schedule is published as three distinct tables, and a family can move between them as children age.
| Which table applies? | Description |
|---|---|
| Children aged 12 and under | All children are aged 12 or under |
| Children aged 13 or older | All children are aged 13 or older |
| Children of mixed ages | At least one child is 12 or under and at least one child is 13 or older |
When a child turns 13, one of three things happens:
- The family stays on the same table (no change to the official cost base)
- The family moves from the 12-and-under table to the mixed ages table
- The family moves from the mixed ages table to the 13-and-over table
Which outcome occurs depends entirely on which child is turning 13 and whether younger siblings remain. This is why parents can see very different changes at age 13 even when incomes and care time are unchanged.
3. Data
The only data used in this article are the official 2026 Costs of Children tables published by Services Australia. These tables define costs as piecewise functions by combined income band with explicit marginal rates.
3.1 "Combined income" bands (2026 thresholds)
- Income Band 1: $0 to $46,569
- Income Band 2: $46,570 to $93,137
- Income Band 3: $93,138 to $139,706
- Income Band 4: $139,707 to $186,274
- Income Band 5: $186,275 to $232,843
4. Methods
4.1 Evaluating income bands
Within each income band, the official tables specify a base amount plus a marginal rate (for example, “12 cents for each $1 over the threshold”). To avoid ambiguity inside bands, we evaluate each schedule at the upper bound of the band. This yields one representative cost level per band.
4.2 Age-transition method
For each scenario, we compare:
- The official cost base before the birthday (child counted as 12 or under, or family still classified as “mixed”)
- The official cost base after the birthday (child counted as 13 or older, or family shifts table)
The percentage change is calculated as:
(Cost after age 13 − Cost before age 13) ÷ Cost before age 13
4.3 Exhaustive scenarios
All real-world cases fall into one of the following six scenarios:
- (A) Only child turns 13
- (B) Eldest of two turns 13
- (C) Youngest of two turns 13
- (D) Eldest of 3+ turns 13
- (E) A middle child of 3+ turns 13
- (F) Youngest child of 3+ turns 13
5. Results
The table below reports the exact percentage change in the official child support cost base when a child turns 13.
Each value is calculated by evaluating the 2026 Costs of Children schedule at the upper bound of each combined income band. The percentages therefore represent the full set of outcomes permitted by the statutory tables.
| Which child turns 13 | Band 1 | Band 2 | Band 3 | Band 4 | Band 5 |
|---|---|---|---|---|---|
| (A) Only child | +35.3% | +40.6% | +33.3% | +24.1% | +24.6% |
| (B) Eldest of two | +10.4% | +10.6% | +11.2% | +10.0% | +10.5% |
| (C) Youngest of two | +9.4% | +9.6% | +10.1% | +9.1% | +9.5% |
| (D) Eldest of 3+ | +9.3% | +9.4% | +9.6% | +9.8% | +9.2% |
| (E) Middle child of 3+ | 0.0% | 0.0% | 0.0% | 0.0% | 0.0% |
| (F) Youngest child of 3+ | +8.5% | +8.6% | +8.8% | +8.9% | +8.4% |
Key takeaways from the results
- Only-child cases are structurally different. When an only child turns 13, the family moves entirely onto the teenagers cost table, producing the largest increase in the cost base (24.1% to 40.6% depending on income band).
- In multi-child families, increases are capped. Where there is more than one child, the increase is limited to around 8%–11% and does not compound with income.
- Middle children do not increase child support. In families with three or more children, a middle child turning 13 produces no change to the applicable cost schedule and therefore no increase in the cost base.
- Income affects size, not direction. Higher income bands reduce the proportional impact for only children but do not eliminate it; for multi-child families, the effect is relatively stable across bands.
6. Discussion
The results show that child support increases at age 13 are driven by table transitions, not by incremental age adjustments.
Only-child families experience the largest changes because the household moves directly from the “12 and under” cost schedule to the “13 or older” schedule. In two-child families, the transition is typically from the “all young” table to the “mixed ages” table, producing a smaller but still measurable increase.
In families with three or more children, the impact depends entirely on whether the child turning 13 changes the family’s age composition. When a middle child turns 13 and younger children remain, the applicable cost schedule can remain unchanged, producing a zero-percent increase. This explains why some families experience a noticeable jump at age 13, while others experience no change at all under identical income conditions.
7. Limitations
This analysis isolates the Costs of Children schedule only. Actual child support assessments depend on additional variables, including each parent’s income share and percentage of care. However, holding those variables constant, the percentage changes reported here measure the mechanical effect of a child turning 13 on the official cost base that is allocated between parents.
8. Conclusion
This analysis demonstrates that changes in child support at age 13 are mechanically determined by the structure of the Costs of Children schedule. The effect is not uniform across families. Instead, it depends on the child’s position within the family and the income band in which the parents’ combined child support income falls.
For families with one child, the transition at age 13 produces a large and unavoidable increase in the assessed cost base. In contrast, for families with multiple children, the statutory design spreads costs across siblings, sharply limiting the impact of any single child aging into the teenage category. In three-or-more-child families, the cost base does not increase at all when a middle child turns 13.
Because the analysis evaluates every income band at its upper bound, the reported percentage changes represent the complete set of outcomes allowed by the 2026 legislation. No larger increases are possible within the administrative system, and no smaller increases are omitted. The results therefore provide a definitive reference for understanding when, why, and by how much child support can change when a child turns 13 in Australia.
