Inverted nipples are normal for both boys and girls and common. Often during puberty the nipples may come out. However, if you become obese they could get worse. There is a minor surgery that I do not recommend. Make sure you consult your doctor.
girls should get a padded bra when they need one ie. when you can see their nipples through their shirt, oor when the are sagging what ever age they are does not matter it matters more if they really need one or not.