Most car seat statutes list more than one number for a stage — an age, a weight, a height, sometimes all three. The number by itself isn’t the whole rule. What matters just as much is how the statute joins them: does a child leave the stage on hitting any one of the numbers, or does the child have to hit all of them?
Across every stage in every state’s data here, this shows up as one of four patterns.
“Any” — the child leaves on the first one reached
This is the most common pattern by far: 24 of the 32 states with a rear-facing rule use it, and 46 of the 51 states use it for the booster stage. Texas’s booster rule is the clearest example — a child is required to use a restraint system “unless the child is taller than four feet, nine inches,” while under eight years old. Read plainly, that’s age eight or 57 inches tall, whichever happens first. A tall six-year-old can leave the booster stage early under that wording; a short nine-year-old stays past the birthday that would otherwise end it.
This is what people usually mean by “whichever comes first,” and it’s worth checking for by name, because the next pattern reads almost the same on the page and means the opposite.
“All” — every listed limit has to be met
A handful of states write a stage’s requirement the other direction: instead of ending on the first limit reached, the requirement continues until every limit is reached. Connecticut’s booster rule requires a restraint for a child “under eight years of age, but not under five years of age, or weighing less than sixty pounds, but not less than forty” — a child has to clear both the age and the weight before the booster requirement ends, not just one.
The wording that signals this is a stay condition joined by “or”: a rule that requires the seat while the child is “under two or under 30 pounds” is really saying the requirement holds as long as either condition is still true, which means the child has to outgrow both before it lifts. Seven states use this pattern for the rear-facing stage, and it’s easy to misread as the “any” pattern above, because the word “or” appears in both. The difference is what the “or” is attached to — a requirement to enter the stage (all), versus a condition for leaving it (any).
“Compound” — a mix of the two
Some statutes can’t be reduced to a single “any” or “all” and combine both within one rule. The District of Columbia’s rear-facing requirement is a good example: a child must ride rear-facing until age two, or until reaching 40 pounds and 40 inches — an “or” at the outer level, with an “and” nested inside one branch. Hawaii’s booster rule works the same way from the other direction: a child ages out at 10 regardless, or earlier at seven years old and over 57 inches tall. Mississippi’s booster rule is a close cousin — a child may leave at seven or by meeting a height and weight combination together.
These compound rules read closer to ordinary conditional logic than to a single age or weight figure, and they’re the ones most likely to surprise a parent who only skimmed the headline number.
“Text” — no numbers to combine
A small number of stages avoid numbers altogether and describe the exit condition as a fit test instead — language like “until the seat belt fits properly.” There’s nothing to combine in these cases; the requirement ends when the seat or belt fits the child correctly, which a store fitting or a certified car seat technician can check, but a birthday or a bathroom scale can’t.
Why the difference matters
The practical stakes are highest for the “any” versus “all” pairing, because the two read similarly at a glance and produce opposite advice. Under an “any” rule, a family can look at whichever number their child is closest to and expect the requirement to end there. Under an “all” rule, reaching one number early changes nothing — the seat stays until the slower-moving number catches up too. Every state’s stage page here labels which pattern applies and, where the logic is a compound mix, spells out the full expression rather than leaving it to a plain-language guess.