At 7–8 years old, the patient is still in the early mixed dentition. By this point only a few teeth have been replaced, and after the four upper front teeth come through there is a resting phase in which tooth replacement stops for about a year. Treatment involving expansion can be started at an early age. A reason for fitting an expander at this age may be a narrow jaw and a crossbite. However, some problems with tooth alignment, and even with the jaws, can correct themselves naturally with growth. For this reason, early orthodontic treatment is only undertaken when necessary.
This answer is for general information. Your own case needs an examination by an orthodontist.