In the context of the scatterplot you described, the following statement is correct:
The input, or independent variable, is years in operation.
In a scatterplot, the independent variable (often the input variable) is typically plotted on the x-axis, while the dependent variable (output variable) is plotted on the y-axis. In your case, "years in operation" is on the x-axis and "enrollment" is on the y-axis, indicating that years in operation is the independent variable and enrollment is the dependent variable.