Is it 12 years or 135cm — which one counts?

Both, and GOV.UK applies whichever comes first. If your child is over 135cm they have reached the height limb even if they are younger than 12. If they turn 12 while still 135cm or shorter, they have reached the age limb.

Can they still sit in a booster after they pass the threshold?

GOV.UK’s child-car-seats page sets the normal duty to use a child car seat until 12 or 135cm, then requires a seat belt. It does not say a booster becomes unlawful to keep using. This article does not add a longer-use rule on top of that threshold.

Does the same 12 / 135cm line apply in a taxi?

The leaving threshold is unchanged. GOV.UK’s taxi exception is about travelling when the driver does not provide the correct child car seat: rear seat only, adult belt if 3 or older, no belt if under 3. Read the exceptions page for that journey.

Who is responsible for getting this right?

GOV.UK’s seat-belts overview places the duty on the driver to make sure children are in the correct car seat until 135cm or 12, then in a seat belt as that page sets out. RoSPA’s law page also states the driver is responsible for children under 14.

Guides · Car seats · The law

When can my child legally stop using a car seat UK?

Updated 10 September 2026 · Checked 10 September 2026 · By MayTally research desk — Checked against official UK sources

What is the legal threshold on GOV.UK?

The child car seats: the law page says: children must normally use a child car seat until they are 12 years old or 135 centimetres tall, whichever comes first. The same page says children over 12 or more than 135cm tall must wear a seat belt.

The seat belts: the law overview repeats the duty for the driver: children stay in the correct car seat for their height or weight until they reach 135cm or their 12th birthday, whichever is first, and they wear a seat belt if they are 12 or 13, or younger and over 135cm.

Swipe sideways for the full table

GOV.UK leaving-the-seat threshold
Child’s situation	What GOV.UK says they must normally use
Under 12 and 135cm or shorter	A child car seat (height-based or weight-based, as that guide describes)
Reached 12th birthday, any height	A seat belt (they may still use a child restraint; the legal child-seat duty has ended)
Taller than 135cm, still under 12	A seat belt (the height limb has been reached first)
12 or 13, or under 12 and over 135cm	A seat belt, as set out on the seat-belts law page

Wording taken from GOV.UK ‘Child car seats: the law’ and ‘Seat belts: the law’, checked 10 September 2026. RoSPA’s Child Car Seats law page states the same 135cm or 12 years threshold. This table is the legal leaving point those pages publish, not a longer-use opinion.

How do I check whether my child has reached it?

Measure height Measure without shoes, standing straight against a wall. Compare the number in centimetres with 135cm. GOV.UK uses centimetres, not clothing-size age.
Check the 12th birthday If they are still 135cm or shorter, the age limb is the 12th birthday. Before that date the normal child-seat rule still applies.
Apply whichever comes first If either limb is already true, GOV.UK’s normal child-seat duty has ended and the seat-belt rule applies. If neither limb is true, they must normally still use a child car seat.
Read the exceptions page if the journey is not a normal car trip Taxis, unexpected short journeys and some other cases are set out on When a child can travel without a car seat. Those pages change the usual rule for that journey; they do not move the 12 / 135cm leaving point.

What happens once they pass 12 or 135cm?

GOV.UK’s line is that they must wear a seat belt. The child-car-seats guide does not say the booster becomes unlawful to keep using; it says the normal duty to use a child car seat has ended at that threshold. Whether a family keeps a high-back in the car after that date is outside the legal leaving question this page answers.

RoSPA’s law page states the same 135cm or 12 years rule and that it is the driver’s responsibility to ensure children under 14 are restrained correctly. nidirect publishes the same 135cm or 12th-birthday figures for Northern Ireland.

Are there journeys where the usual rule is different?

Yes — GOV.UK lists them separately. They are exceptions for a trip, not a new leaving age.

Taxis and minicabs: if the driver does not provide the correct child car seat, children can travel without one only on a rear seat — with an adult seat belt if they are 3 or older, and without a seat belt if they are under 3.
Unexpected journeys: a child aged 3 or older can use an adult seat belt if the journey is unexpected, necessary and over a short distance and the correct child car seat is not available. GOV.UK says you cannot take children under 3 on that kind of trip without the correct seat unless it is a licensed taxi or minicab and they travel on a rear seat without a seat belt.
No room for a third child car seat: children under 3 must still be in a child car seat (in the front if the back has no space). Children aged 3 or older can sit in the back using an adult belt.
Minibuses and coaches: GOV.UK sets different ‘if available’ rules; coaches and minibuses are not the same as the family car.

Exception wording is summarised from GOV.UK ‘When a child can travel without a car seat’, checked 10 September 2026. Read that page for the full conditions before treating a trip as exempt.

Height versus age — which comes first?

GOV.UK’s phrase is whichever comes first. A tall eight-year-old who is already over 135cm has reached the height limb. A shorter child who turns 12 while still under 135cm has reached the age limb. You do not wait for both.

The child-car-seats guide also says you can choose a seat based on height or weight while they still need one. That choice is about the seat they use *before* they leave, not a second leaving date.

What this page does not decide

This page only restates the GOV.UK leaving threshold and the main exceptions. It does not compare approval labels, and it does not tell you to stay in a booster after 12 or 135cm. If you need those topics, use the official guides for labels and fitting, not this leaving-date question.

In shortGOV.UK: normally a child car seat until 12 or 135cm, whichever comes first, then a seat belt. Measure height, check the 12th birthday, apply the first limb that is true. Exceptions live on the GOV.UK journey page. The threshold is the legal leaving point, not a longer-use verdict.

Questions parents ask

Is it 12 years or 135cm — which one counts?: Both, and GOV.UK applies whichever comes first. If your child is over 135cm they have reached the height limb even if they are younger than 12. If they turn 12 while still 135cm or shorter, they have reached the age limb.
Can they still sit in a booster after they pass the threshold?: GOV.UK’s child-car-seats page sets the normal duty to use a child car seat until 12 or 135cm, then requires a seat belt. It does not say a booster becomes unlawful to keep using. This article does not add a longer-use rule on top of that threshold.
Does the same 12 / 135cm line apply in a taxi?: The leaving threshold is unchanged. GOV.UK’s taxi exception is about travelling when the driver does not provide the correct child car seat: rear seat only, adult belt if 3 or older, no belt if under 3. Read the exceptions page for that journey.
Who is responsible for getting this right?: GOV.UK’s seat-belts overview places the duty on the driver to make sure children are in the correct car seat until 135cm or 12, then in a seat belt as that page sets out. RoSPA’s law page also states the driver is responsible for children under 14.

Sources

Official pages first. If this page and the official page disagree, the official page wins.

GOV.UK — Child car seats: the law — checked 10 September 2026
GOV.UK — Seat belts: the law — checked 10 September 2026
GOV.UK — When a child can travel without a car seat — checked 10 September 2026
RoSPA — Child car seats: the law — checked 10 September 2026
nidirect — Child car seats, restraints and seat belts — checked 10 September 2026

This is information, not a guarantee of any payment, and not medical, legal or product-safety advice for your home. Prices and rules change; figures carry the date they were checked. Recall status comes from official records only — a product we cannot find is not a safety all-clear. Tell us if we got a fact wrong.