Recently I had a few papers published, and I suppose in response to this, I received a request from Zentralblatt to be a reviewer. They ask some general questions about what I would feel comfortable reviewing, and if I would be willing to receive papers electronically. My concern about just signing up stems from the fact that I am only a second year graduate student. So my question is

When is the right time to start reviewing?

and

When did you start reviewing for journals/reviewing organizations?

Thanks ahead of time!