Given an instance of the STUDENTS relation as shown below: For ($\textit{StudentName}$, $\textit{StudentAge}$) to be a key for this instance, the value X should NOT be equal to _________.

A set of attributes forms a key for a relation instance if and only if no two distinct tuples have identical values for that set of attributes (uniqueness property).

Examining the tuples in the given relation for $(\textit{StudentName}, \textit{StudentAge})$:

1. $(\text{Shankar}, X)$

2. $(\text{Swati}, 19)$

3. $(\text{Shankar}, 19)$

4. $(\text{Swati}, 18)$

5. $(\text{Ganesh}, 19)$

If $X = 19$, the first tuple becomes $(\text{Shankar}, 19)$, which is identical to the third tuple's $(\text{Shankar}, 19)$. This would violate the uniqueness property required of a candidate key.

Therefore, $X$ should NOT be equal to 19.