"Bishops in their 30s" were actually quite common right before Vatican II ... especially in mission territories, where people needed the Sacraments. And that's the case today as well. Now, certainly, no bishop would receive ordinary jurisdiction over a diocese at such a young age, but then that's not what's going on here either. We are in a missionary situation these days. I don't have any real problem with the age. One might argue that there are too many bishops, but not compared to the size of the world, and the "two bishops in every garage" is an exaggeration. It's more like two bishops for every continent.
There is a joke going around that for the sedevacantists, there are more bishops than priests around
Thục-line bishops
The "Thục-line" bishops essentially means bishops who derive their episcopacy from Archbishop Thục or from bishops in Thục's lineage. The "Thục-line" is lengthy and complex, reportedly comprising
200 or more individuals.[37] Many bishops in the "Thục-line" are part of the non-sedevacantist but conclavist Palmarian Catholic Church; this is due to Thục having consecrated Bishop Clemente Domínguez y Gómez, future head of the Palmarian Church, and the very numerous episcopal consecrations within this organization.
In 1981, in Toulon, France, Archbishop Thục consecrated three sedevacantist bishops. On 7 May 1981, he consecrated the sedeprivationist French priest Michel-Louis Guérard des Lauriers as a bishop. Des Lauriers was a Dominican theologian, an expert on the dogma of the Assumption, an advisor to Pope Pius XII,[38] a former professor at the Dominican university Le Saulchoir in Belgium and later at the Pontifical Lateran University in Rome,[39] a former professor at the International Seminary of Saint Pius X of the SSPX in Écône, Switzerland, and the main intellectual force behind the famous critical study of the Mass of Paul VI (Novus Ordo Mass) called the Ottaviani Intervention, presented to Paul VI in October 1969.[40][41][42] On 17 October of the same year, Thục consecrated the two sedevacantist Mexican priests and former seminary professors Moisés Carmona and Adolfo Zamora as bishops.[43] Carmona and Zamora had been sedevacantist leaders and propagators in Mexico[44] for many years,[45] and were among the priests who formed the Unión Católica Trento[43] (Tridentine Catholic Union).[31] The Vatican declared Thục ipso facto excommunicated for these consecrations and for his declaration of sedevacantism.[46]
The sedevacantist community generally accepts and respects bishops descended from the three sedevacantist bishops Thục consecrated in 1981 (des Lauriers, Carmona, and Zamora).
Living notable sedevacantist bishops who descend from Archbishop Thục through Bishop Moisés Carmona include Bishop Mark Pivarunas of the Congregation of Mary Immaculate Queen (consecrated in 1991 by Carmona), Bishop Daniel Dolan of Cincinnati (consecrated in 1993 by Pivarunas), and Bishop Martín Dávila Gandara of the Sociedad Sacerdotal Trento (consecrated in 1999 by Pivarunas and Dolan).
Living notable sedevacantist bishops who descend from Archbishop Thục through Bishop Guerard des Lauriers, O.P., include Bishop Geert Stuyver of the Istituto Mater Boni Consilii (consecrated in 2002 by Bishop Robert McKenna, O.P.), Bishop Donald Sanborn of the Roman Catholic Institute (consecrated in 2002 by McKenna), and Bishop Joseph Selway of the Roman Catholic Institute (consecrated in 2018 by Sanborn, Dolan, and Stuyver).
Méndez-line bishops
On 19 October 1993, in Carlsbad, California, United States, Bishop Méndez-Gonzalez consecrated the sedevacantist Father Clarence Kelly of the Society of Saint Pius V (SSPV) to the episcopacy. By Méndez' wish, the consecration was kept secret until his death in 1995.[47][48][49]
There are two sedevacantist bishops who descend from Bishop Méndez through Bishop Kelly: Bishop Joseph Santay (consecrated in 2007 by Kelly[50]) and Bishop James Carroll (consecrated in 2018 by Santay and Kelly[51]).
Kelly, Santay, and Carroll are bishops of the Congregation of Saint Pius V.
Whose lineages derive from earlier movements
A considerable number of sedevacantist bishops are thought to derive their holy orders from Bishop Carlos Duarte Costa, who in 1945 set up his own schismatic "Brazilian Catholic Apostolic Church".[52][53] Carlos Duarte Costa was not a sedevacantist, and instead questioned the status of the papacy itself – he denied Papal Infallibility and rejected the pope's universal jurisdiction.[54] In further contrast to most Catholic traditionalism, Duarte Costa was left-wing.[55]
Source:
https://en.wikipedia.org/wiki/Sedevacantism