2013 Fermat Problem 2

The graph shows the mass, in kilograms, of Jeff's pet Atlantic cod, given its age in years. What is the age of the cod when its mass is 15 kg?

\textbf{(A)}\ 3\quad \textbf{(B)}\ 7\quad \textbf{(C)}\ 4\quad \textbf{(D)}\ 6\quad \textbf{(E)}\ 5

If there are no answer choices shown, enter a numerical answer.

Full credit to this problem is given to the CEMC, you may view all Fermat contests here.