The answer is:
The old-age dependency ratio is calculated as the number of people who are age 65 years or older, divided by the number of people age 15 to 64, times 100.
The old age dependency ratio tells the number of those people who are aged above 65 year as a share of those between 15 to 64 years.
In the other words, this ratio tells us how many retired people one worker will have to sustain.That is to say this ratio tells us the number of people who are not earning anything rather they are dependent upon other people for their day to day.