To express Justin's age in terms of Kendra's age, we start with the information given: Kendra's age is half of Justin's age.
Let \( a \) represent Justin's age. According to the problem, we have:
\[ \text{Kendra's age} = \frac{1}{2} \times \text{Justin's age} \]
Since Kendra is 13 years old, we can write:
\[ 13 = \frac{1}{2} a \]
Now we need to find the equation among the options that correctly represents this relationship. Rewriting the equation gives us:
\[ \frac{1}{2} a = 13 \]
This matches the first option. The correct equation that represents Justin's age \( a \) is:
\[ \frac{1}{2}a = 13 \]