4ish months, once she was stable & could support her own head/back
6 months because she didn't get good head/neck control until then. She's nearly 9 months and still loves it
4ish months as well for me! He loves it and has some strong legs to hold him up.
I had my 4 month old in his but my doctor told me to keep away from it until he can sit up by himself because you can cause them to start toe walking if you use one to early so I just put mine away for now until he can sit himself up unassisted
4 months ive had my LO in from in short burst. His development with toys has been great since learning to play wiyh all the individual parts