This varies from gym to gym, but overall you will see a broad range of ages in all xcel levels. The oldest age group at our silver xcel state meet this year was 14 1/2 and up, so plenty of "older" gymnasts!
My DD16 was 12 when she was a Silver -- turned 13 a couple of months after the season ended. She was often in the oldest age group at meets but was never the oldest gymnast there. That's one of the perks of Xcel!