Answer
13.48 years, seems to be good.
Work Step by Step
By using the values from the picture and the given formula:
$s=\sqrt {\frac{n\cdot[\sum(f\cdot x^2)]-[\sum (f\cdot x)^2]}{n\cdot(n-1)}}=\sqrt {\frac{121\cdot 889150.25-10244.5^2}{121\cdot 120}}=13.48 \ years.$ This value is more than 3 other data from other exercises, but only slightly more than the largest, so seems to be good.
