When they can sit up almost unassisted so around 4-6 months. It's also not good to use for more than 10-15 minutes at a time because it's hard on their hips.

bouncer like a jump a roo or an exersaucer? I say they would be ok as long as head control is good .. the walker I think my son was like 7 months old, as long as they can sit up, assisted pretty well so they are concentrating on using their legs and not trying to just sit up

I think the bouncer will be okay.