Everyone develops at different rates. My step-daughter developed the need for deordorant anti-perspirant at the same age. I think it is completely appropriate for her to use it. This doesn't have anything to do with age. This is a hormonal change. Some of my friends didn't begin their periods and develop breasts until late middle school. The girls in my family received their menstration around age 11. I wore a size C cup at 11 years old. I was an average size child and considered thin. Everyone is different.