Look at a church of any size, and you'll see they are handling age groups properly.  They have classes for definite age ranges, such as children who 3-5 years, 6-7 years, and 8-10 years. There are Juniors, Middlers, High Schoolers, College-age and all the rest.

This is exactly as it should be because age group characteristics are part of the child development process. Three year olds learn in one way, 13 year olds in another way. Thier interests are rather different too!