Merck Manual

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Introduction to Growth and Development


Evan G. Graber

, DO, Sydney Kimmel Medical College

Last full review/revision Apr 2021| Content last modified Apr 2021
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Physical growth is an increase in size. Development is growth in function and capability. Both processes highly depend on genetic, nutritional, and environmental factors.

As children develop physiologically and emotionally, it is useful to define certain age-based groups. The following terminology is used:

  • Neonate (newborn): Birth to 1 month

  • Infant: 1 month to 1 year

  • Young child: 1 year through 4 years

  • Older child: 5 years through 10 years

  • Adolescent: 11 years through 17 to 19 years

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