Our parish has new servers (first year), middle-rank servers and lead servers. There are typically four servers per Mass. Every Mass has to have a lead server (that is, if you are scheduled as a lead, you have to find a lead to substitute for you) and at least one lead server or very experienced middle-rank server for every new server. There are also servers who are incense-trained and servers that have been trained to do bells. That training and becoming a lead is by invitation, but essentially everyone progresses along, provided they even want to be lead servers. (A habit of not showing up when scheduled would result in not being asked to be a lead server.) There is also an enforced dress code that is more strict than the adults would accept for themselves, to be blunt. (No jeans and also no shorts except in exceptionally hot weather, for instance.)
Yes, we are blessed to have exceptionally dedicated altar servers. Some remain in altar serving from fourth grade all the way through high school. We sometimes run a little short, but it is more common for there to be a waiting list. The parishioners are constantly telling the altar servers how much they are appreciated, though.