What month you start putting your child in a juperoo?

I have a 4 months old baby. I'm a first time mother so i really don't know anything about babies. My mil is still in the old time. I brough my baby a Fisher-Price Rainforest Jumperoo , MIL doesn' like it . She think that the baby is still to young to be in it. She say that if i let her stand on her feet she'll devolp, what thats call bug legs or something like that . ( forgot the name) She think its not good for her back, she might devlop bad posture. I don't know if its true or not. So i was wondering, what month did you guys start putting your child in to a jumperoo? I think its good to put her in now , so she could work her muscle. If im wwrong please tell me.