Yes it was serious. Here it is again:
Ballotpedia: The Encyclopedia of American Politics
ballotpedia.org
Election policy
Ramaswamy's campaign website said, "Vivek supports a constitutional amendment to implement Civic Duty Voting amongst Americans aged 18-25. In substance, this amendment will increase the standard voting age to 25, while still allowing all Americans to vote at age 18 if they meet a national service requirement (at least 6 months in the military or a first responder role) or else pass the same civics test required of naturalized citizens.