Sydney was 3 months old when she first started going in hers. She didn't spend too much time in it, because her head wasn't that sturdy yet, but the more we put her in there, even just for a few minutes at a time, helped her with her balance. We would just sit right in front of her and help her out for the first little while.
Now, she's 5 1/2 months and she loves it. She's just starting to learn that it's her bouncing, so she's always just a goin'.