The percentage of all Americans who are younger than 65 years old can be calculated by adding the percentage of Americans in the 0 to 14 years segment and the 15 to 64 years segment.
Percentage of Americans 0 to 14 years old = (18/100) x 100 = 18%
Percentage of Americans 15 to 64 years old = (65/100) x 100 = 65%
Total percentage of Americans younger than 65 years old = 18% + 65% = 83%
Therefore, 83% of all Americans are younger than 65 years old.