JMHO, Confirmation should be given as early as possible, and not used as a carrot to extend participation in church or as some sort of milestone. It feels coerced. A priest once told me that teens need the sanctifying grace bestowed in Confirmation before they hit their teens.
But the one good thing about confirmation latter on is that it helps the teens at a point in life when they start to make important decisions about what their state of life and careers are going to be with the help of the Holy Spirit.