Acquiring ‘Milestones’ at regular stages of child growth is known as :

Developmental psychology distinguishes several related terms for how a child changes over time: growth (a quantitative increase in physical size), maturation (the internally-driven unfolding of biological readiness), skill development (practice-based refinement of a specific ability), and developmental tasks -- the age-linked achievements a child is expected to reach at a given stage of growth.

Sitting up, walking, and talking are examples of such milestones -- each is expected to appear within a defined age range. The standard pedagogical term for "milestones acquired at regular stages of growth" is therefore developmental tasks.

• Maturation names the biological readiness process itself, not a specific age-linked accomplishment.

• Growth names the quantitative change in a child's size -- height and weight -- not a milestone achievement.

• Skill development names the gradual, practice-based refinement of one specific ability, not the expected age-linked milestone as a whole.

Since the question asks for the term naming milestones reached at particular stages of growth, the answer is Developmental Tasks.