What to tell a 7-year old

Recently, my cousin frantically posted on Facebook looking for input about how to talk to her daughter, who, if I recall correctly, is in second grade, so about seven years old. To quote her “What age is best to start talking to your daughter about ”girly” changes that she will go thru? Some minimal changes are already starting to take place and I am also unsure how to start the conversation when it is to happen. HEEELLLLLLPPPP!!” What kind of resources can I point her toward? Thanks for any help here.