What would the independent variable be for this class lab?

Okay, so an older person and a younger person had to open their hand as many times as they could in 20 seconds and repeat this 10 times until obviously, there was a burn in their hand. The test was to see if the older or younger person could open their hand more. What would the independent variable be?





Thanks for any help :]|||The age of the person ("older" or "younger").