According to Jean Piaget, the ability to use symbols — such as letting the…

Jean Piaget described cognitive development as a sequence of four stages — sensorimotor, preoperational, concrete operational, and formal operational. The formal operational stage, emerging from about age 11–12 onward, is marked by abstract, hypothetical, and symbolic thought: children become able to reason about symbols that have no physical or tangible referent, such as a letter standing for an unknown quantity.

The stem describes exactly this capacity — using a letter such as X to stand for an unknown numeral is a purely symbolic operation with no concrete object behind it. This is the hallmark abstract-reasoning skill of the formal operational stage, and it is this same capacity for manipulating undefined symbols by formal rules that underlies a child's readiness to learn algebra and calculus.

• concrete operational stage + algebra and calculus: During the concrete operational stage, logical reasoning is tied to tangible, physical objects, so it cannot yet extend to an undefined abstract symbol like a letter standing for a number.

• concrete operational stage + conservation and seriation: This correctly names the achievements of the concrete operational stage (conserving quantity across an appearance change, and ordering objects by a dimension), but these are not the abstract symbol-use skill the stem describes.

• formal operational stage + conservation and seriation: This correctly names the stage of abstract thought, but conservation and seriation are mastered earlier, in the concrete operational stage — they are not the abstract-symbol capability introduced with formal operations.

So the developmental stage is the formal operational stage, and the skill it enables is algebra and calculus.