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
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"
“You touch a creature and stimulate its natural healing ability.”
If stem cells could solve the issue, it’s possible. Turn it into a surgery, high DC medicine check, or an insane arcana check and a DM would probably let it play.
You couldn't "restore" something to a state it's never been in. Cutting off the legs and then casting regenerate would "regenerate" the lost pair of paralyzed legs.