My proposal is to complete this project of your specifications, done and ready to run within 5 days.
The program will deal with everything you need it to, i.e. average age, as well as error handling. Can add Extra things as you need.
I think to implement this I would create a doubly-linked list of all your family members where you can then access the information, such as name, age, etc. I can also implement this in any method you need.
Who am I? I will be a junior next year at a top end university studying computer engineering and computer science, I've been learning java for the past year and have recently began to pick up C.
If you have any questions feel free to ask