That strike allowed the Paraguayan wunderkind to eclipse the previous record of 17 years, one month and 28 days held by Sergio Diaz.
Nestor Camacho equalised for Olimpia 12 minutes later.
Ovelar, who is two months and two days away from his 15th birthday, was substituted 14 minutes into the second half of a game that summed up the excitement and chaos of Paraguayan football.
Marcos Acosta thought he had stolen the points for Cerro with a goal five minutes into injury time but then Olimpia controversially made it 2-2 with a penalty eight minutes later.
Both sides also had a man sent off moments from the final whistle.
Leaders Olimpia remain seven points clear of their rivals at the top of the Paraguayan first division.
(Reporting by Andrew Downie, editing by Pritha Sarkar)
