My youngest has a delay and her physiotherapist advised strongly against both jolly jumper and walker. The problem with jolly jumpers is that the Jolly Jumpers encourage muscle imbalances, since they put the baby/child in a sitting or upright position long before their young developing spines and nervous system are ready for it.
Walkers are considered rather unsafe cause they can gather much speed so you child can tip over, fall and injure him/herself. When you do want to use a walker make sure to weight it down with something heavy

We didn’t have a jolly jumper. Mine went into a walker at 4 1/2 to 5 months of age. They took a little while to work out how to use it, but once they got it, they were off!