Even with regenerate, what exactly are you regenerating? If the necessary neural pathways for the legs to work never developed in the first place, they couldn’t be “regenerated”. If this was your goal I think you might need to true polymorph a guy into “the same guy but his legs work”

It’s explicitly within the capabilities of a Lesser Restoration, but also I would not allow a player to cast that spell on another player if that other player didn’t want it

Edit: also as another person said, the adult who has never used their legs before never learned how to walk, so even if they had functioning legs, it would not help