The definitions and examples of key statistical terms are summarized as follows:
- Population Parameter: \( \mu \) (Mean age of all students in Boston)
- Sample Statistic: \( \bar{x} \) (Mean age of 200 randomly selected students in Boston)
- Variable: \( X \) (Age of an individual student in Boston)
- Observation: \( x_i \) (Age of a specific student in the sample)
Thus, the final answer is:
\[
\boxed{\text{Population Parameter: } \mu, \text{ Sample Statistic: } \bar{x}, \text{ Variable: } X, \text{ Observation: } x_i}
\]