Any age! United Methodists practice an "open table." We do not exclude people based on age or religious tradition.
We do understand that the concept of Communion is a tough topic (even for adults!). For this reason when the Communion servers get to children, they may not say the formal phrase "The Body of Christ ..." but rather a more age-appropriate phrase such as "This means that God loves you very much." Younger children are literal, and talking about the "body" of Christ and eating is weird! Instead, focus on this wonderful traditional ritual of our church. Help your child feel comfortable even when standing in line. Quietly explain what is happening. If you are going to pray after being served, prepare your child for this quiet moment by telling him or her, "Let's pray for a second." (But keep it quick for their sake.)
My own daughter called Communion "Snack Time" in worship. Not an altogether bad idea - it is a community of God's children gathered for a meal, just like in Sunday school.
Communion is taught throughout the years in the Sunday school curriculum as well as extensively during sixth-grade Confirmation.