We have a relatively contemporary set up, and rotate worship leaders between 3-4 people, with rotation of instruments and singers. There are times we'll have the congregation sing a capella (with the praise team singers) but perhaps sometimes play too much (I'm likely the worst offender!)
On the topic of ages at which we involve children, is the powerful impact on children and youth of inviting them to participate as singers on the praise teams. One high-schooler at her Council interview prior to Profession of Faith last month, when asked what was the greatest spiritual influence for her, and what had helped her grow most, said unequivocally and immediately that it was singing with the praise teams. Last Sunday we had one other adult and me, along with two high-schoolers, a junior high-schooler, and three elementary age kids. Congregational response was very positive on several levels. We usually have at least one or two younger children singing with us, without condition on their musical abilities coming in - very occasionally we'll have them sing into switched off mikes if they're off-key. But they usually grow into the role, are active participants in worship leading, and generally find it rewarding while creating cross-generational unity. We also invite them to join in readings, prayer, etc. As leaders we talk with them about worship, why we're doing what we do, the themes and subjects of the songs, etc.
Given all the challenges of retaining our younger members after they go off to college, I'm convinced that early involvement in worship is a vital element and a wonderful way to integrate all ages together.
To answer your question "At what age do you foster worship leading?" we have several grade schoolers participating in the choir with their mother on some of the songs. We also have highschool students in our choir. Finally one of our regualr worship teams is lead by high school students and young adults. It is so uplifting to see their love for God shining through as they lead us in worship.
To help our younger children participate in worship we have made some small flags for them to use. It's interesting to see some of the older children beginning to catch on to how to use these to worship. We plan to continue to provide appropriate education about what the use of the flags is for.