Stroller specifications state a maximum weight and often a height, rarely an age. That convention comes from how the products are tested and what actually determines whether a limit has been exceeded.
Testing measures load, not birthdays
Product safety standards for juvenile products specify structural tests: static and dynamic loading, stability, restraint strength and repeated use cycling.
Every one of those tests is performed with a defined mass. The stated limit is the load the frame, folding mechanism and restraint were verified against.
Age has no role in that verification because age is not a force. Two children of the same age can differ enough in mass to matter structurally.
Height limits address a different failure
Where a height limit appears, it usually reflects geometry rather than strength. A child too tall for a seat sits with the harness positioned incorrectly.
Center of gravity is also affected, since a taller occupant raises it and changes stability, which is the property tested for tipping.
Head clearance under canopies and bars is a further constraint. A stroller can be well within its weight limit and still be outgrown.
Who writes the standards and who enforces them
Juvenile product standards in the United States are developed through voluntary consensus standards bodies with participation from manufacturers, testers and safety advocates.
Federal regulation makes certain of those standards mandatory for durable infant and toddler products, so compliance is not optional for products in that category.
Manufacturers must certify compliance based on testing by accredited laboratories, and registration cards exist so that a recall can reach owners.
Accessories and cargo are part of the load
Storage baskets and parent trays carry their own separate limits, and bags hung on handles are outside the tested configuration entirely.
Hanging weight on a handle changes the tipping behavior the stability test measured, which is a common real-world failure that no limit on the seat covers.
Second seats, boards and adapters are only within specification when the manufacturer lists them for that model. Cross-brand combinations are untested arrangements.
Where to check rather than assume
The manual carries the model-specific limits, and manufacturers publish current versions online for products whose paperwork has been lost.
Federal recall listings are searchable and cover juvenile products, which matters particularly for secondhand purchases where the original registration never reached the buyer.
For questions about a particular child's positioning or readiness for a given seat, a pediatric clinician is the appropriate person to ask.