Verifying a person's age
The biggest problem I see here is that you are printing a warning message to System.out and then using a default value for something which sounds like an Exception.

Why allow negative values at all?

if (initialAge &lt; 0) {
    throw new IllegalArgumentException("Initial age cannot be negative, was specified as " + initialAge);
}
age = initialAge;


This way it is always up to the caller to pass a valid age instead of the method defaulting to zero which will bring other problems. It is better to have your program break early because of a bug than to have a warning message be printed out which may indicate a bug.