At a typical celebration of the ordinary form of Mass two lay ministers really are required: one person to read the preliminary Bible readings and another to serve by bringing books and vessels to the celebrant when needed. Sometimes people are surprised to discover that there are rules against having priests and deacons do these things, but when you stop to think about it this makes sense: if Mass is supposed to be a cooperative effort of the whole community, it’s wrong for just one or a few persons to usurp all the action!
Ideally, a separate reader should proclaim each Bible passage; only the Gospel is reserved to a deacon (or, if there is no deacon, to any assisting priest who is concelebrating). One who reads the Scriptures at Mass, of course, should not only read well but also be a person who treasures the Bible as central in his or her personal spirituality. At Mass a reader also can do some of an absent deacon’s tasks, like leading the litany in the penitential rite, noting the intentions of the “Prayer of the Faithful” after the homily, and making announcements or commentary to the congregation.
Altar servers have many auxiliary roles at Mass: carrying censer, cross, and candles in procession; holding the Missal for the presider when he prays at his chair; helping the deacon or priest “set the table” when the gifts are collected and prepared; etc. Although for centuries it’s been permissible and customary for youth to help in this way, this doesn’t mean the role is not important. Not only what a server does but the reverent way which he or she carries out those duties add a great deal to the whole experience of the Mass.