A population parameter describes an entire population; a particular characteristic of an entire population. A value that describes the entire population. In this context, the population parameter is looking into the average ages of fathers when they had their first child, but this value is not given. The mean given here is the sample statistic.
Since, the population studied here is a very large population (all fathers), you have a statistic. But sometimes, if the population is very small and groups are small enough to measure, you have a parameter.
