I/13 x (where x is the age)
Explanation:
Let the age in consideration is 'x' in variable
It is mentioned in the question as 1/13 the age
so, 1/13th the age be 1/13*x
hence the algebraic expression to be written as x/13 or 1/13*x
Here 1/13 is multiplied with x, the age in variable.