The most misread two characters in a toy shop are "3+".
It is not a judgement about difficulty, and a child playing happily with a 3+ toy at two is not ahead of schedule. In the United States the marking is tied to regulation on small parts: a toy intended for children under three cannot contain a part that fits inside a small-parts cylinder — a test object about the size of a young child's throat. Manufacturers who do not want to meet that requirement label the product 3+ instead.
So the label usually answers one question: does this contain something a child who still mouths objects could choke on?
Two consequences follow.
Reading it as a difficulty rating gets it backwards. Plenty of 3+ toys are trivially simple and carry the label only because of a small wheel or a detachable figure. Plenty of 18-month toys are genuinely demanding.
It says nothing about your particular child. The regulation is about a population, not a person. A cautious four-year-old who still puts things in their mouth is the child the label was written for, whatever the number says.
There is a second kind of label — the one that suggests a developmental stage, sometimes with a month range. That is marketing rather than regulation, and it is worth reading with the same eye you would bring to any other claim on a box.
The home version of the test does not require a cylinder. If a toy or any piece of it drops through a toilet-roll tube, it is not for a child who is still mouthing things — regardless of what the box says.
The reverse case is worth stating too, because it is the one that makes parents anxious. A confident three-year-old handling a toy labelled for older children is a judgement call about that child and that toy, not a rule being broken. The number on the box was set by a manufacturer deciding which test regime to meet. You have information it does not: whether this particular child still puts things in their mouth, and whether anyone smaller is sharing the floor.