Our system uses September and it varies on what level a boy can compete. At 6, he can compete in L4; 7 -L5 and so on. It's not necessarily done by grades. So you could have a 12yo L4 and a 6yo L4. L8 is a step to L9/10 which is consider Junior Olympic Elite as it uses the FIG system but is modified a bit. All the routines from L4-L7 are compulsory, so they do not make their own routines but they have option for certain bonus skills (except on vault).
4'5 69lbs isn't overly big. One of my L4's was about that size and age. While he was nearly as tall as his 10yo brother, I didn't get the impression he was overly big for his age.