In my research we measured heart rate variability parameters (4 different numerical variables) in 4 different situations in 50 subjects - newborns who were of different age. How to control the sample for the age? I would like to see if the situation itself independently of their age effects variability parameters? Thanks

In my research we measured heart rate variability parameters (4 different numerical variables) in 4 different situations in 50 subjects - newborns who were of different age. How to control the sample for the age? I would like to see if the situation itself independently of their age effects variability parameters?