…Second question: Are Masses that exclude groups of people, such as a “youth Mass” where only teenagers in high school are allowed to attend, licit?
The Church has a document which specifically endorses Masses for particular groups of people. It is the 1969 Instruction
Actio Pastoralis, “On Masses With Special Groups”. It includes:
“…That is achieved by the shared celebration of the eucharist, especially on Sundays and holydays, around the bishop and within the assembly of the parish, whose pastor is the bishop’s representative.
But pastoral care also has as its purpose and direction particular groups. This is not for the sake of creating
ecclesiolae or privilege, but to serve the faithful’s particular needs or to deepen the Christian life in accord with the requirements and capacities of the members of these groups. This brings to the advantages that spring from a special spiritual or apostolic, common bond and from the desire to help one another toward spiritual growth.
From experience pastoral activity teaches how much good having their own celebrations can do for these groups. For when these celebrations have proper and wise planning and direction they are no obstacle to parish unity; instead they serve the parish’s missionary activity by their power to achieve closer contact with some of the people or to deepen the formation of others. …”
(From Documents on the Liturgy 1963-1979, Liturgical Press, Minnesota, 1982, ISBN 0-8146-1281-4, pages 672-673.)
Examples of groups of given in the document: “groups gathered to make a retreat”; “meetings of any sector of the parish for the purpose of pastoral activity”. The Mass may take place in “private homes or institutions” if approved by the Ordinary.
Another document on this issue is the 1973 Directory for Masses with Children. Chapter 3 has the heading “Masses with Children in Which Only a Few Adults Participate”. It begins:
“20. In addition to the Masses in which children take part with their parents and other family members (which are not always possible everywhere), Masses with children primarily in which only a few adults take part are recommended, especially during the week. …”.