I believe the wording is such that you have to be eligible for POTUS to be VPOTUS. 35, natural citizen, etc. Bill Clinton was reportedly trying to find a way to get on a ticket and since he’s a constitutional scholar decided he’d not win a challenge
Such a stipulation doesn’t exist for speaker or other positions in the line of succession. You could be speaker of the house at age 25 since that’s the requirement to be a member of the house.