Welcome to YT! Most females will come into heat between 7 to 10 months old. My Jasmine didn't have her first heat until she was 18 months old! 
Is she small?
If you really have concerns, speak to your vet. 
__________________
Joanne and Mickey, Rocky, Wilma, Pebbles, Winnie, Jasmine, Sammie, Peanut, Emma, Rambo |