When would it be advised to divide by n as opposed to n-1 , and under
what conditions could someone use this method to determine the
If you have the data of the (whole) population, then you divide the sum of squares by n. The definition of the term "population" is defined/explained here.
The population could be the member of a community. And the property could be the age. If you have all the data of the ages of the members, than you don´t have to estimate the standard deviation. You just calculate it. n in the denominator.
If you don´t have all the data, then you make a sample of size n. From this sample you can calculate an estimator for the standard deviation. This estimator is unbiased, if you have n-1 in the denominator, instead of n.
The sample values should be drawn independetly and with replacement.
Does 5σ represent a data point that is five standard deviations away from
If you mean $\overline x+5 \sigma$ or $\overline x-5 \sigma$, then you are correct.