The >40% is of households. Children typically don't have their own households. And retirees can still pay income tax, both because social security is taxable and because they're making withdrawals from tax-deferred retirement accounts. It's not at all uncommon for higher income retirees to pay more income tax than lower income working people (as it should be, but the point stands that the retirees don't explain the total).
That's only for determining who gets certain tax deductions. Minors still have to pay their own taxes and file their own tax return (if they make more than the threshold for filing) even if they are claimed on their parents taxes. The only way you can have multiple people paying or filing taxes together is if they are married.