Euclid's first axiom states that the things which are equal to the same thing are equal to one another.
It is given that, the age of A is equal to the age of B and the age of C is equal to the age of B.
Using Euclid's first axiom, we conclude that the age of A is equal to the age of C.
Thus, Euclid's first axiom illustrates the relative ages of A and C.
Hence, the correct answer is option (a).