How early can a baby use a walker
Older walkers are actually frowned upon these days because of the risk of rolling down the stairs with them. We did have one with our oldest and then got rid of it, like many things that took up space and we could live without.
Otherwise the more modern walkers like exersaucers (sp?) etc. I'd say when they are holding their heads up well and starting to stand in your lap, they can be used for short periods of time.
As far as toys that assist in walking, as soon as they are pulling themselves up and taking steps. Though be careful of the ones they can pull down on themselves if they fall down.