At our gym the step 4 (similar to level 4) girls are about 8 to 11. I think this is the norm for most of the city clubs. In the smaller towns, I think there is a wider age range, up to teenagers, competing in the lower steps. The steps program was designed with this in mind, so there are two age divisions for competitions, 'under' and 'over' (I think for step 4 it is 'under 10' and '10 and over'). A few gyms also have international development program (IDP) for the most talented girls, which has its own levels. Some of these girls who are only 8 or 9 are training the equivalent skills of step 6 or higher.
Having said all that, I think our gym has recently opened up competitive opportunities to more girls of a wider age range to get more gymnasts coming through the steps program. Over here we compete right from step 1.