Answer:
7.07 years
Explanation:
The standard deviation of a population is easily calculated by a spreadsheet or calculator. Care must be taken to use the formula for population standard deviation, rather than the formula that assumes the values are a sample.
__
The attached calculator shows the standard deviation is about 7.07 years.
_____
Additional comment
The population standard deviation is the square root of the variance. The variance is the mean of the squares of the values, less the square of the mean of the values. The second attachment shows the calculation done this way.
The units of standard deviation are the same as the units of the members of the population. Here, that is "years."