I think a lot of having a younger team vs. older team depends on the gym's level structure and coach's preference. For instance at our gym, we don't start competing until old 4/new 3. Kids in the preteam are chosen by the head coach/owner, who prefers a young team. He picks mostly 4-6 year olds for the preteam, so our old 4/new 3 team tends to be very young.
We have about 20 new L3s this season, and almost all of them are 6 and 7 years old.
On our new L4 team, most of the girls are 7 and 8 years old. We are usually the youngest looking team at the meets we attend. Sometimes the youngest age groups are made up of only our team.
I have a very young new L4 (she is 6, but will turn 7 in time to compete in November) and while good form/tightness/toe point comes naturally to her, it is taking her a lot longer to memorize all the little details of the routines when compared to the older girls on the team. My older DD (9) is also a new L4, and she is memorizing the routines easily, but form is not natural to her. If I had $1 for every time I've heard the coaches telling her to fix her form, I'd be rich.