The phrase "average adult" refers to a typical or common person who is fully grown. It is often used when making generalizations or comparing statistics. Full definition
Up front are comfortable seats for average adults, though taller drivers will be short on headroom. (kbb.com)
Average adult dogs need about 18 percent of their calories to be from protein. (animals.howstuffworks.com)
For decades now, scientists and experts from all branches of knowledge insist that average adults need 7 - 9 hours of sleep per night in order to function optimally. (fitnessandpower.com)