The function checkAge should only take in the minimum amount of information needed to perform it's job. Adding anything else just creates an artificial dependency. If only an int is needed then that is the solution I should take.
I would argue that in this case, the answer is probably neither. Either "Age" would be factored out into its own class, or if the operation is context-specific with Person, it would be found inside the Person class itself.
Follow the law of demeter for functions. Basically the law states that entities should be loosely coupled. Ask yourself the following question, should a computer object know about a person object? In this case, maybe all you are doing inside checkAge is checking an int value. If that is the case then what makes you think passing the entire object is needed? Simply pass the person age and take it in as an int in this case.
So prefer
public void checkAge(int n)
with the information given, neither solution is good
the first solution requires the Computer class to know about Person.Age, for no apparent reason
the second attaches a method to the Computer class that has nothing to do with the properties of the Computer object
some context would be helpful - if this is a validation, then CheckAge belongs to the Person class (with possibly an IsAgeAcceptable property)
Why is the Computer checking the age of a Person? The answer to this determines what makes sense...
I would like to point out that when passing a reference, the reference is a 32 bit integer while a data type is copied. So if your value type is larger that a 32bit int, pass by reference if performance or memory is any sort of issue.