You have not successfully initialized either of your two local variables. sumAgesSquared was simply not given an initial value. Since sqrRootDividedbyTwo is initalized as sqrRootDividedbyTwo= Math.sqrt(sumAgesSquared)/2 that also fails.
age is not an array. You are sending multiple parameters to your function but only capturing the first one in a variable age. If unsure of the number of parameters you’ll have to access all of these using the arguments object.
Consequently, age.length is undefined because of above.
I would suggest starting with multiple parameters issue and then tackle the uninitialized local variables afterwards. Use the console to verify the results of your changes.