No
You don't understand.
The income decreases, for example, my sister no longer received child benefit, and as soon as her son started working, the household income meant she lost housing allowance and council tax reduction.
So her income went down by the child benefit amount + housing allowance. Bit her rent remains the same AND she's paying 100% council tax.
(Made up figures, but will illustrate the issue...)
Before 18:
Wage £1000 + £500 benefits
Household income totals £1500.
To pay rent of £500 and Council Tax of £75. Total cost £575.
Leaving here £925 for the month.
She still has to pay, water, gas, electric, internet, insurance, travel, food etc. all "fixed" costs. Totaling £700.
Wih the remaining £225, she had to clothe her child, pay for haircuts, childcare, kitchen appliances, shoesetc.
Now (because another adult is earning)
Wage £1000 ... No benefits
Contribution from adult children £0.
Rent still £500, council tax is now £100. Totals of £600.
Leaving her £400 a month's for everything else. Obviously no need for childcare etc, but
She just can't afford the fixed costs.
So yes, adults that are earning money need to contribute.