As far as I know, the VPOTUS (Vice President of the United States) has no age limitations. With that, could a 30 year-old VPOTUS serve as president under the line of succession in the event the POTUS is killed or incapacitated?
The Twelfth Amendment states that "no person constitutionally ineligible to the office of President shall be eligible to that of Vice President of the United States." Thus, to serve as vice president, an individual must: Be a natural-born U.S. citizen; Be at least 35 years old.
Because the age requirement for President is 35 years old, the Vice President must also meet that requirement.