I'm not absolutely sure, but I would advise waiting until your baby can hold his head and neck very well on his own. I've also read that the child should be able to sit by himself. I know the company says to use 'til max. 28 lbs. or walking. Whichever comes first... Hope this helps...
Here's a link to advice about this exerciser. They mention that they're not recommended, but I know tons of people who've used them safely. I share this link for the advice about siblings which I think may be useful.
https://www.chw.edu.au/parents/kidshealth/safety_factsheets/pdf/jolly_jumpers.pdf