Each rotation is assigned an "age group" of kids. This changes from one cruise to the next based on the primary ages of kids on the ship (cruises during the school year have more toddlers/pre-schoolers....those on school breaks have more older kids).
That said, one rotation will be primarily families with little children, one will be families without children and with older children/teens and one will be families with "in between" children. Realize that this is a very inexact science as many families have kids in multiple age groups. Early seating has many more young children than late.
All that said, if you do not make a request, you will be assigned to the rotation that best fits your family composition. If you do make a request, you will probably be assigned your request. Just be aware that the request may put your teen with the toddler group, or your toddler with the "old people" group.
We always request our favorite server regardless of his rotation...and have never found that to be a problem at the late seating. I have personally developed an aversion to early seating since my DD has entered teen years....I find it to be much more noisy and hectic.