Category:Young adults in the 1970s by country

1. Babies (birth to 24 months)
2. Children (2–13 years [puberty])
3. Teenagers (13–17 years)
4. Adults (18+ years)
   • Young adults (18–39 years)
   • Middle-aged people (40–59 years)
   • Old people (60+ years)

Definitions come from, but are slightly modified from, the Physical stages of human life as found at Wikipedia:Human development (biology).