Early childhood educators are now recommending mixed-age groupings as a better learning environment. Older children learn from being in a leadership role, and younger children benefit from the example of an older child.
The classes have more of a "family feel" rather than a school orientation. Siblings can participate together, as well as moms, dads, grandparents, and caregivers.
The curriculum is designed to teach on many levels of complexity at once.
Children of the same age are not necessarily in the same stage of musical development.
The activities are parent-child oriented, not child-child, so children don't need to be with children of the same age to benefit.