Swimming competition encourages teams to fill the lanes and give more experience to the swimmers. Even if an age group cannot fill the pool, a team can fill the lanes with younger swimmers. This is called "swimming up."
This is done simply to score points! It is not meant to challenge younger swimmers to swim more competitively.
A swimmer may swim up one age group in individual events and two age groups for relays.
This is often done to put together more possible relays, and occasionally, the coach will put a swimmer in a higher age group if we are lacking in swimmers for that age group.
We do this simply to try to score points and hopefully win the meet -- working towards winning our Division!