To answer the original question, it's generally recommended to wait until 1 year to start agility classes (and widely recommended to wait even longer for larger breed dogs). That said, if it's an agility foundation class, then she'd be fine doing that now. Kwik is almost 7 months and I'm just starting on foundation training with her. We work on things like tunnels, running through jumps with the bars off, contacts (she stands on a wooden platform about 3 inches high and puts two feet on the floor and two feet on the platform), and ground work (rear crosses, front crosses, blind crosses). Things like that that don't require heavy impact on their bodies are fine, and would actually help a great deal if you ever decide to compete.

I also highly recommend you make sure she has a really good stay. Keeva always had an amazing stay but it slowly got worse and worse as she got more excited about agility.