What is an estimator and how to construct it? The definition of an estimator "rule that tells how to calculate an estimate " as given here is not clear to me. If I make measurements of some quantity, say age in a group of N people, my outcomes will be $x_1, \dots, x_N$ where $x_i$ represents the age of $i$-th person. One can now talk about the mean as the quantity that gives us "rough idea" about the age of this group.
I want to understand how is the notion of estimator relevant to a situation like above (if at all it is)? What the estimator actually is and how to construct it?

The mean age would be the estimator, and the mean value would be an estimate, a statistic from the example you mentioned.
