DBMS Superkey indentification

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DBMS

Hi all,

I am doing some revision for a database module and I have come across this question in a past paper:

Identify all the superkeys in the following table:

ID	First name	Last name	Age
1	Sean	O'Connor	22
2	Tim	O'Connor	27
3	Jack	Black	32
4	Jack	Coventry	3
5	Natalie	Spifflington	27

Now the answers say that the super keys are:
{Employee Number, FirstName, LastName, Age}
{FirstName, LastName, Age}
{FirstName, LastName}
{Employee Number}

My question is why isn't say (lastname, age) a superkey? As far as I have understood superkeys there is no tuple of lastname, age that has two different entries.

Thanks in advance to anyone who can offer some insight.