5. There is a sequence of numbers $a_{1}, a_{2}, \cdots, a_{m}$, and $S_{n}$ represents $a_{1}+a_{2}+\cdots+a_{n}$. If $a_{1}=1005$, and $S_{n}=n^{2} a_{n}$, then the value of $a_{2000}$ is ( ).
(A) $\frac{1}{2007}$
(B) $\frac{1}{2008}$
(C) $\frac{1}{2009}$
(D) $\frac{1}{2010}$
5. C.

From $S_{n}=n^{2} a_{n}$, we get $S_{n-1}=(n-1)^{2} a_{n-1}$.
Subtracting the two equations and rearranging, we obtain
$$
a_{n}=\frac{(n-1)^{2} a_{n-1}}{n^{2}-1}=\frac{n-1}{n+1} a_{n-1} \text {. }
$$

Therefore, $a_{n}=\frac{n-1}{n+1} \cdot \frac{n-2}{n} \cdot \frac{n-3}{n-1} \cdots \cdot \frac{2}{4} \cdot \frac{1}{3} a_{1}$
$$
=\frac{2}{n(n+1)} \times 1005 \text {. }
$$

Thus, $a_{2009}=\frac{2010}{2009 \times 2010}=\frac{1}{2009}$.