You are working on a project that involves processing a large dataset using Java. You need to utilize the Function interface and streams to efficiently manipulate the data and produce specific outputs. To ensure that your team has a thorough understanding of these concepts, you decide to create a challenging programming question that tests their knowledge and problem$-$solving skills.

Consider a scenario where you have a dataset containing a list of employees in a company. Each employee object has attributes such as name, age, department, and salary.

Design a Java program that uses the Function interface and streams to perform the following operations:

Develop a program that reads the dataset and stores it in a collection.

Write a function using the Function interface that takes an employee object as input and returns the employee's name and department as a concatenated string.

Using streams, generate a new collection that contains the concatenated strings obtained from the previous step.

Enhance your program to find the average salary of all employees using the stream's built$-$in functions.

Generalize your program by incorporating a filter function that only includes employees whose age is above a certain threshold $($e$.$g$.,30$ years$).$

Summarize the program by explaining the purpose, characteristics, and usage of the Function interface in Java, highlighting how it represents a function that takes an input and produces an output.

Compose a complete Java program that implements the given scenario, demonstrating your understanding of the Function interface, streams, and their practical applications in data manipulation.