Is u[-n] defined from negative infinity to zero as it is folding of u[n] or from negative infinity to -1?
$u[n]$ is the unit-step function :

$$u[n] = \begin{cases} { 1 ~~~,~~~n \geq 0 \\ 0 ~~~,~~~ n &lt; 0 } \end{cases}$$

then 

$$u[-n] = \begin{cases} { 1 ~~~,~~~ -n \geq 0 \\ 0 ~~~,~~~ -n &lt; 0 } \end{cases}$$

or stated as
$$u[-n] = \begin{cases} { 1 ~~~,~~~ n \leq 0 \\ 0 ~~~,~~~ n &gt; 0 } \end{cases}$$

Hence, $u[-n]$ is nonzero for the range of $n \in (-\infty,0]$.