In Ch. 6 we learn that Lily hates her hair ribbons and we also learn that she will only have to wear them for another year. Lily is a 7 in Ch. 6, but will soon be an 8. At 9 she will no longer have to wear the hair ribbons, which little girls wear. By not having to wear them anymore, we can assume that the community views the age of 9 as one where young girls begin to move from childhood to adulthood.
We also know that all 10's , boys and girls, have their hair cut short. "Girls lose their braids." We often think of pig tails as a little girl hair-do, and in the community of The Giver they are no different. Hair ribbons are simply there to decorate the pig tales. Think of them like a bow or a headband. Older girls and women do not wear those same hair accessories.