Take, for example, a double-income couple with two teenagers living in California, one of the high-tax states where SALT deduction limits could throw off withholding calculations. They earn a combined $300,000 and deducted $29,000 in SALT, $16,000 in mortgage interest, and $7,000 in charitable contributions on their 2017 tax return.

Though this family gets hurt by the SALT limit, they benefit from changes to the alternative minimum tax, or AMT. Under the old withholding rules -- under which the family withheld a relatively high amount, claiming no personal allowances -- they’d still end up writing a check of almost $4,000 to the IRS each year, because they were hit by $6,500 in extra taxes from the AMT.

In 2018, the new withholding tables should boost this family’s take-home pay by $8,426, according to estimates by the Tax Institute at H&R Block -- a noticeable $702 more per month. They also no longer need to worry about the AMT, which was sharply limited, though not eliminated, by the new law.

Their final bill next April, however, could vary widely based on a factor not reflected on their old W-4s and also unrelated to the SALT and AMT changes -- the age of their children.

The former W-4 counted all dependents equally, reflecting a $4,050 personal exemption for every person on a tax return, from toddlers to college-age kids and elderly relatives. The tax revamp eliminated personal exemptions, so the new W-4 must make distinctions between children under the age of 17, who are eligible for an increased $2,000 tax credit, and other dependents who only get a $500 credit. For withholding purposes, then, a child is worth four times the value of other dependents. The law also made the child tax credit available to more upper-income taxpayers.

If the California family’s children are 15 and 16 years old, H&R Block estimates, they’ll get to April and find they owe the IRS $2,758, 30 percent less than last year. However, if their kids are 17 and 18 -- ineligible for the child tax credit -- they’ll need to write a check for $5,773, almost 50 percent more than last year. They’ll even need to pay the IRS a small underpayment penalty of $15.

‘Tax Scam’

The vast majority of U.S. workers will see some tax cut as a result of the law, at least initially. Though the law’s benefits for individuals fade over time, 65 percent of American households can expect a tax cut in 2018 and 6.3 percent will see a tax hike, according to estimates from the Tax Policy Center.

Democrats deride the law, which they’ve branded the “tax scam,” as a giveaway to the wealthy and corporations that offers relatively small, temporary benefits to middle-income taxpayers. In a January letter to the IRS, top congressional Democrats raised concerns that the IRS’s withholding tables might be intentionally skewed to boost workers’ pay now and leave them owing money in 2019, after the midterm elections.

Treasury Secretary Steven Mnuchin said worries about political motives were “ridiculous.” In a letter obtained by Bloomberg News to Senator Ron Wyden, an Oregon Democrat, the IRS said it would “help workers ensure they are not having too much or too little withholding taken out of their pay.”