Age intervals from the video are 18-25, 26-35, 36-45, 46-55, 56-65, and 65+. These are intervals with lower and upper limit but their scale or level of measurement is ordinal.
Data or variables which are of interval level have can be added and subtracted. A popular example is temperature. A 20 degrees added to 10 degrees Celsius is 30 degrees Celsius.
This addition and subtraction cannot be done with the age intervals. They are categories that can be be ordered from lowest to largest or vice versa.
Hope this helps.
The 365 Team